I don't play with N18, so not sure whether it's defined there. But I believe it is indeed the case for some GW systems (not sure which - Warcry perhaps?) that models are not friendly to themselves. So, it is plausible, even if counterintuitive.
(Personally, I think it would be simpler if friendly did include the model in question. It would be easy enough to say other friendly models if the model was supposed to be excluded.)