I don’t see the upside for Mueller to leak this letter to the NYT. It makes him look ineffective and weak ... which is why I suspect it might have been leaked from the Trump side. LOL.
I think that if Trump leaked it, he would have said he did, like he did in the recent past.
He is not kind of person that would let Mueller take the rap for something he didn’t actually do.
Everyone else, including Mueller, would do something like that, but not Trump.
Trump’s people are not going to leak to the New York Times.