JandK, you got a bit confused about relations here... an Intellect devourer is not supposed to care about thralls at all. They are just thralls a-ka slaves. While our characters are tadpole bearers, his masters' children.
Plus subsurvient is not exatly same thing as loyal. It looks as if Larian used it in archaic meaning here. And that archaic meaning is "not being able to resist". You're cripling it to be able to kill it more easily "should it pose a threat".
