Eh. I don’t think this is a convincing argument. It makes the assumption that a time traveling civilization would value bitcoin. Who’s to say time travel isn’t theoretically be invented in a post-bitcoin world, or by a non-human civilization somewhere in the universe?

The absence of evidence is not the evidence of absence. Saying that because something hasn’t happened yet, it’s clear time travel could never be possible simply isn’t a compelling argument. You could make that argument about a bunch of things.