Yes, but think about what happens when you try to shoot an arrow through thick vegetation. The vegetation doesn't disappear so your arrow can go through, then wave and say "Have a nice time, dear!"
Basically, obstacles exist for both attacker and defender. Try to think of it this way: If it can hinder your opponent, it can hinder you. It's good to hide behind, but you can still get hit through it. It's good as a way to stay hidden and still be able to attack, but it's harder to attack from there than out in the open. I like to run the game with lots of options and possibilities, but there are consequences for every choice. Not always bad, of course, but they're there. I'm also running with the descriptions out of the adventure. There is nothing that would indicate that the cover and concealment only apply if you're getting attacked while in/behind the foliage. In fact, as I quoted before, it specifically says they exist if you attack into it, which you effectively are.
Of course, had you targetted the wyrmpriest or the skirmisher, they have lighter armor, and you probably would've hit even with the cover/concealment. An enemy called a 'dragonshield' and described as heavily-armored is
probably harder to hit.