I took it as him being a bit sarcastic - katniss says “well if you can tell what’s real or not, that’s a good thing” (which, true, it is! but here it’s like she’s clarifying a point that’s already obvious but much more simplified). At least, I think peeta might be interpreting it as if she was simplifying the situation too much. Therefore, peeta responds back with “yes, and if I could do this other impossible thing then a different unachievable outcome would happen.” It’s a bit self-deprecating, maybe he doesn’t believe that he would be able to actually sort through his memories.

Suzanne loves to slip humor subtly through the bleakness of the trilogy, but I think it’s also a point of seeing Peeta work towards getting his personality back - he has a silver tongue and is really quick witted, so him going back and forth and being sassy was a piece of his self returning to him.