What if they didn’t want us to know they were from the future? Seems like anyone that was capable of to-the-past time travel in the first place would also be slick enough to disguise themselves if they wanted.
Freegards
People don't understand that the theory of relativity says something about the geometry of our universe. If that geometry is incorrect, then the basic laws of electricity and magnetism do not work (the electromagnetic field is possible only from a correct, relativistic understanding of the classical -- and later quantum mechanical -- fields; at the time James Clerk Maxwell summarized the laws of the classical electromagnetic field, he had already written down the theory of relativity. We wouldn't know that for almost 50 years.)
In addition to electrodynamics, none of quantum field theory works without relativity. There would not -- and could not -- be antimatter since this is a prediction of the Dirac equation, which is based on the Special Theory of Relativity. We know antimatter exists, with the properties predicted by a Lorentz covariant theory. That alone guarantees that the universe has the geometry that it has, and there can never be FTL travel as a result.