I believe this is because he pled Guilty to some of the charges...as part of the deal, these others were dismissed.
We don't know what else it means---but as it is more complex than 2+2=4, there are going to be Partisans who think they know exactly what it means. He will co-operate against Somebody---if you are rational (as in Republican) you expect it to be Manafort; if you suffer from TDS (as in Democrat), you believe. you want to believe...it will be about Trump.
Since the dude was only indicted for the things he did with Manafort before Trump became a candidate, Manafort is the best guess.
But if you have TDS, you are sure its Trump.
This is how we will entertain ourselves for awhile. And one of these Parallel Realities which is apparent on this Board will be very disappointed.