He's heard that. He doesn't exactly believe it, because a) it makes no sense, and b) if they could, wouldn't they have better things to do with the technology? Interdimensional time travel for the express purpose of harassing the people they've kidnapped -- it's stupid. (Brock theorizes it's more along the lines of recording their memories of events, which at least makes sense.) Still, she's letting it go, so he does, too.

"They're still sending kids," Brock says in the most gentle tones he can muster; he doesn't really want her to buy into the propaganda that they're somehow indispensable just because they're not from here. "It's easier for them to dehumanize us this way, but don't you believe it for a second."