Of course he did....he wants a cabinet post
You are exactly right. I'll give him one thing; he's pretty clever at walking a tightrope.
Now that he's given his nominal support for the hag, I expect the actual energy he expends to be minimal, unless by some nightmare, it becomes obvious that she'll be a runaway winner.
He'll be pretty flat in support, because then he hasn't thrown out the chance for a cabinet position in a Republican administration either.
It's become the "in thing" to have an "moderate" opposition party member at least discussed for something or other. Secretary of Defense seems to be one of the most common. Bush II tried to get Breaux, Obama did get Hagel.
His "support" for Hillary could be "excused"... and may even be helpful to a Republican ready to tout his inclusiveness.
Manchin is a lot of things, but dumb ain't one of them.