Actually, you're correct, and my original answer was wrong. That's what I get for writing at 2am.

Here's how it works generally: when you plead guilty the judge warns you that they do not have to accept the plea deal and can sentence you however the hell they wish. You plead guilty and then the judge tells you if they accept the prosecution's deal. I've seen several defendants surprised by the judge not taking the sweet probation deal and turning around and giving the defendant years in prison which they are unable to appeal.

So, in theory, the judge could potentially give Assange some time.