This letter is also mentioned in a book by A.N. Wilson on the Shroud of Turn, as he tries to trace the history of the shroud. He proposed that following Jesus’ resurrection, the apostle Thomas took the shroud with him to Edessa, where it remained hidden for several centuries. He mentions this letter in passing.
I suppose that it is not absolutely impossible that there could be some truth to the story, but as pointed out by others, there is no other corroboration of it, except, of course, by Eusebius.
And even if the letter were authentic, that would not make it part of divine revelation or binding upon Catholics. It would be like a private revelation.