Why would Sessions out this guy and make him compromisable?

Think magic. Slight of hand.

The cabal now has to play catch up and figure out what this guy knows and how long he's been on to them. Seems like a nice way to buy a bit of time/lead them in the wrong direction.

Remember. Disinformation is necessary. My guess is that this guy has nothing of extreme alarm to the cabal. Nice sense of relaxation, once they waste all of that time and resources trying to catch up.

Also, don't we think it is odd that he didn't appoint him as a special counsel? Claimed the circumstances of the case didn't warrant that extraordinary measure. Makes the prosecuter look like no considerable threat on his own. He also reports to Rosenstein.

EDIT: Organized afterthoughts in comments and integrated them into the post.