You're suggesting that Coyote never actually believed his theory at all, and he just said it to break up their friendship? I can get behind that theory, it fits pretty well with him being a trickster and a huge jerk.
The question is, what if anything does he get from that? Maybe he is just threatened by the fact that Ysengrin has a friend at all. Many abusive spouses hate that because it feels like a loss of control(or something?).