This is going to be difficult to articulate. I'm going to fail to articulate this in a way that other people understand. People are going to continue to assert that the only way to compare two things is to make them have all the same stuff (feats, skills, items). While there is some scientific validity to this, involving confounding variables, in that the addition of wealth, items, and skills that are more powerful than Monk class features do make this analysis more difficult. However, I'm still going to explain why this logic is flawed, so that everyone can just tl;dr and carry on with this discussion to the umptienth power. Good luck with that.
Let's start here then shall we? How many D&D games that in no way involve items or money are there in comparison to the games that function in the way described in the player's handbook and dungeon master's guide where wealth and item acquisition are a major role? Clearly then, not using items at all is completely nonsense because no games will be played under this constraint, so the ability of a naked monk with no skills and combat feats to beat up a naked expert with no skills and combat feats is hilariously irrelevant. Similarly, if you note, the tier list states that the classes are hypothetically carried to the same level of optimization, that would include proper item choice and proper feat choice.
Firstly, I would like to assert that Class skills are as much a class feature as anything else, in just the way that HD per level is a class feature. We could all agree that a Fighter with everything the same, but with a lower HD (perhaps a D4) would be a weaker class. But it would be a weaker class not through HD? I think we could also agree that if Fighters had 20 skill points per level and had all classes as class skills they would be stronger than they are now. Skill points are clearly then a function (one might say feature?) of a class. The problem then, and I assure you it is only a problem with how people FEEL the game should work rather than how it DOES work is that some skills are simply more powerful than the features of a class. That may offend someone's sensibilities but it continues to be the case. Use Magic Device is a perpetual offender because it gives you the ability to cast spells, for money. Most of us can agree that cast spells is one of the most powerful class features, particularly if it is full casting which Use Magic Device can do. It again, can offend the way some people think the game should work, but that's just the truth of it. Is that the only reason that Experts are better than Monks in the specific case? No. Monks have close to ZERO meaningful class features, just like Experts do and have worse skills.
I want to give another example. Imagine comparing these two classes, which have the class feature "Magic" and no other class features. They have the same HD (d4) and 0 skill points per level.
Weaker Wizard
Magic:Weaker Wizard characters can cast spells exactly as a Wizard of their level casts spells. Weaker Wizard characters do not get a feat at first level.
Wizard Take 2
Magic: Wizard Take 2 characters can cast spells exactly as a Wizard of their level can cast spells.
Can we then agree, that the second Wizard is more powerful? It can cast spells in the same way, but receives one more feat. Would your mind be changed if Weaker Wizard characters got a +1 to Spot with a class feature called "Canny Insight"? I hope not, because with that feat one of the weakest things you could possibly do is get +2 to Spot, among other things. Clearly then the class features of Weaker Wizard are not worth the lost feat. If we were then to extrapolate this to Expert vs Monk, and simply look at Use Magic Device, the argument can be made that the Monk can just take a feat to get the ability to use Use Magic Device as a class skill. Are it's other class features then worth a feat? Maybe? Do they still knowing that Experts have other class skills that are quite powerful? They can also use Psionics, Autohypnosis, Lucid Dreaming, Handle Animal, etc. That may not jive with your imagination of what it should be able to do, but has little bearing on a discussion of their relative power levels.
An argument has been made for item standardization as well.
At level 20 I will give every class in the PHB these items:
Vorpal Greatsword +5
Heavy Fortification Full Plate +5
Standard Adventurer's Gear
Holy Symbol
Heward's Handy Haversack
Material Component Pouch
There, so now we have a list of items to make every class "fairly balanced". We will remove skills, because everyone seems to assert that they are not class features. No one may have other items. We won't use feats either, because anyone could take them! (lol?) In alphabetical order then:
Barbarian: Can wear the full plate and sacrifice some class features, but likes the sword. The rest of the stuff is pretty whatever!
Cleric: Can wear the plate, can use the sword with a penalty I suppose? Likes the Holy Symbol! Casts spells and wrecks everyone.
Wizard: Doesn't use any of those items except the component pouch. Still manages to wreck the Barbarian and he and the Cleric walk off in to the sunset.
Etc.
All we've proved is that the power level of the Wizard and the Cleric is so far above the Barbarian, it doesn't even matter that they are naked. What about if we compare a Monk and a Barbarian? The easiest way for us to image is just to imagine the two fighting, but a one on one fight with no other terrain or factors is pretty rare in game, so that's sort of foolish. The next easiest way is to imagine them in different situations. They fail at nearly all except direct combat ones, in which case the Barbarian is pretty clearly ahead, with actual useable items and class features that make him a better fighter (instead of making him better at jumping off cliffs I guess? Or making him not take penalties for aging which will never come up, or making it so he is immune to things that only affect Humanoids). If you then picture him fighting an expert, or the two in a fight, again the barbarian wins out.
But an Expert (or any class that has features that do not strictly revolve around combat) might be built that avoids combat all together. Many wizards spend a good deal of time avoiding physical combat, and fair fights. Still Wizards are clearly good, and clearly better in combat than many other classes. There is more to D&D than just combat and the wizard can shine there too, and I don't mean RP either or other games that all everyone to talk their way out of anything. Plenty of situations call for non-combat abilities. I think the worst thing that could be concluded is that they are equal, even if monks "shouldn't" be equal to experts.