I'm thinking they went with UNIX timestamp because they assumed the source would always be another note.
Discussion
or they didn't think about the problem of how much public domain literature there is that overflows even negative unix int64 timestamps - it's 1970-2140 for 64 bits - 2037 for 32 bits, so i think if you allow negative ones to mean before UNIX epoch then it couldn't be more than 1800 before it flips to ~2140
nah, in fact, relays can and do reject events with funny timestamps, certainly, auth timestamps "should be" constrained to a short window of a minute or less in order to stop replay attacks