Jamie is about to protest that it would be a very good idea for the Time Lords to not be able to call the Doctor back, but then he has to go and mention explosions. And even if Jamie's unclear as to what anti-matter is (although he can take a guess), large explosions aren't good in general.

"Aye, I suppose not," he says, somewhat disappointed. It would have been nice to not have to deal with the CIA and then they could just go do what they wanted. Get Victoria, maybe...

His thoughts are interrupted when Damon continues, and he's hopeful again for a moment - and then he's somewhat confused. "Why would the Doctors on the Plane know?" he asks, trying to puzzle it out. "Because they're later regenerations?"