This is assuming that the bitcoin time chain is widely accepted as a source of immutable truth. Despite us believing it to be so, the majority of people pre-2009 don’t view gold as the dominant store of value, despite that factually being the case. The thing about the concept of “truth” is that ultimately, it’s subjective because it relies on the beliefs of people who recognize truth and have shared opinions with others who recognize the shared truth.
What if the civilization that invents time travel doesn’t in aggregate see bitcoin as the immutable truth? What if they’re communicating telepathically through implanted hardware, making lying impossible and as such, there is no need for a neutral system to establish and enforce “truth”?