He’s chosen to stay silent on others. He hasn’t been set up in any way.
I would assume he’s chosen this because it’s likely there would be many embarrassing things, and possibly even more crimes uncovered, if everyone involved had to start giving their evidence publicly.
He’ll do a couple of years in jail, come out and no doubt be paid handsomely for selling his story. Possibly even be looked after by the many people he has protected (given her role in the party I actually I think there will be others far more complicit than NS - many others had much more oversight of the finances than her).
He’s actually choosing to not share the whole story with the Scottish people.