Thomas Jefferson wasnt even in the country at the time the first amendment was written.
He was in France.
He's not talking about Jefferson writing the 1st Amendment, but rather he is desperately trying to use the letter as evidence of intention. Not that it matters, since the phrase or clause he's trying to invoke is not in the Constitution OR the Bill of Rights, or, for that matter, in any of the subsequent amendments. But he won't let that stop him.