I think that there are other strange things in the first book. For example, the woman escaped riding on Cassie. So, there is a woman who knows everything about the Yeerks, and she's free. What about her? Did the yeerks try to kill or recapture her?
Remember that crazy woman from MM#1? That woman is widely believed to be the same woman that Cassie rescued.
Anyway, about the subject at large, I'm guessing that the reason nobody suspected that the Animorphs were human is because Visser Three is a batsh** lunatic (as has been mentioned in the books as well as in this thread). I can easily picture him responding to "Hey, I think maybe the Andalite bandits are human?" with <Are you implying that I am so incompetent as to be stifled by mere human youths!?!> Fwap!
That's the sound-effect for an Andalite's tail, for anyone with bad memory, by the way.So it makes some amount of sense to me, without invoking the "Ellimist did it" clause. It isn't that no Yeerk ever realized the Animorphs were human. It's just that any Yeerk who came forward with that information quickly unrealized it.