Took me a while to see this after it was mentioned. The post from Trump and the letter are definitely odd... Perhaps I’m falling into the trap of reading too far into things but this isn’t clear on its face, perhaps I need to review previous firings from Trump more.
It is eerily similar to Sessions...like a relay runner. Appear to have done nothing, a Twitter admonishment by Trump, then the hand-off. I think they are each playing a vital role and once that is done the next level players take over. Just odd that Barr's letter clearly does not say retirement, resignation, or anything of the sort. More and more it seems like a move on a chessboard.