"Alternate timelines happen all the time, that's how the world works. But Earth— Earth is still herself in all of them" So even if inhabited by different people, some things do stay the same... Gaia, Alaya, whatever you decide to call that entity. Parallel timelines with differences are something Caster's familiar with. So he is not surprised by such a claim, in fact, he considers it a high possibility as well.

"I don't really feel old, but there is no denying that I died around 2000 years earlier than the current year" or at least the current year in his latest manifestation. It's unknown which year it is on the train.