Derek very much means 'we' as in pack, including Stiles, but he's not about to say anything about that when the boy so clearly isn't interested in that. Which doesn't change the fact that he is pack, even Isaac can probably feel the shift in the pack bond by now, even if he's not quite sure what it is or means, but Derek's not about to risk the inevitable rejection if he made that clear to Stiles. This is good enough for now, and the fact Stiles doesn't protest Derek's presence means he can continue to protect him.

"I don't have a clue what they're called, but I'm pretty sure it's not that." He can't quite keep from snorting in amusement, even if he doesn't want to encourage Stiles... or maybe he does. "And if we do get an infestation they'd probably go after you just for that."