That's a recommendation for a lesser sentence, or more likely an agreement not to contest the defendant's request for a lesser sentence. the judge can accept that, or not (ask Jared Vogel, the former Subway pitch-man, about that). But, if he lied while testifying, the government will come down on him.
Again though -- crooks hang out with crooks. It isn't all that big a deal that Gates is a POS. He's a rat, and I agree that nobody likes rats. Doesn't mean that Manafort isn't a tax cheat.
Link: https://www.youtube.com/watch?v=UiXvWl9pJjc