At heart Joe Manchin is a liberal. He votes for their side on anything that will not hurt him politically at home in conservative coal state West Virginia.
Voting green may sink Joe Manchin in the next election. He barely won his last election. I suspect Joe Manchin may not desire to run again thus his vote.
So you're suggesting Manchin is throwing in his hand? If so, then everything he's done up to now to save his own political future was for naught? Or maybe they've just funded the "Manchin retirement account" in the Caymans, and he got word 'The Check cleared'?