We're talking about a signature that's published in a public database. The camera's timestamp doesn't matter, just the database's.

That proves it existed at a specific time in the past, not that it didn't exist before that. What's stopping a hash of the Mona Lisa on a block chain with today's date?

Ah, I thought you were saying the hash proved something on its own. Lots of weird ideas about crypto in this thread.

Physical access means all bets are off, but it's not required for these attacks. If it's got a way to communicate with the outside world, it can get hacked remotely. For example here's an attack that silently took over iphones without the user doing anything. That was used for real to spy on many people, and Apple is pretty good at security. Most devices you own such as cameras with wifi will likely be far worse security-wise.