They were too far out for their tractor beam, because they missed due to the jump inaccuracy. The Breen dropped out of warp much closer to it, and were able to snag it with theirs.
On rewatch, I wonder if he was on the way to suggesting someone like Book when he was interrupted
Agree with this, it seemed like he was using Book for context, not for a real suggestion. Saru respects what Booker has done for everyone, but I don’t think he’s a real big fan of the guy, and certainly doesn’t think he should be the XO of a Federation ship.