Spells=Ex...
The words "Does Not Compute" come to mind. And yes, I am aware of that argument.
Note that Spells were actually an Na Special Ability back in 3.0 (MMII), before becoming Ex in 3.5 (in 3.0 you could have Na Special Abilities, in 3.5 you can't). But yeah, we've been through it a bunch of times. Spells is specifically defined as a Special Attack (which is a subset of Special Ability) in the Monster Manual. All Special Abilities are Ex, Sp, or Su according to the same (which is the primary source on Ex, Sp, and Su abilities), and this statement is echoed in the PHB, DMG, Fiend Folio, SRD, and a bunch of other books, in addition to the Rules of the Game Article on Polymorphing (which goes into types for quite a while). I can quote a dozen books all listing Spells as being separate from Sp and Su abilities, and the definitions of Sp and Su abilities show how they're not the same as spells. That leaves only Ex as a possibility. Furthermore, you still have your Spells ability in an Antimagic field, so the ability to cast spells itself is not magical, even though the spells themselves are (much like how Wildshape is a magical Su ability that grants non magical Na abilities like claw attacks). And bringing it home, MMV and one of the Fiendish Codexes both list a special ability to cast spells as Ex (though note that they call it a Special Quality there).
So, yes. Spells are Extraordinary Special Attacks (possibly also Ex Special Qualities... the two aren't necessarily mutually exclusive). This is a subset of Special Abilities. If you think that's broken... well, house rule against it. I certainly would (I don't tend to allow Polymorph anyway).
JaronK