Well, Ax is pretty much an Andalite teenager, so he was probably just exaggerating, like kids do.
My favourite KASU, though, is in Animorphs 18: The Decision, when Ax claims a Seagull has talons. Although, perhaps this is remedied in Michael Grant's Gone: due to the FAYZ mutating everything that lives inside the bubble, Astrid finds a Seagull that has grown, you guessed it, TALONS. So, either Applegrant know very little about Seagull biology, or they're predicting an inevitable evolution in seafowl.