From my quick read, it appears that:
1 -- the DA entered into some sort of non-prosecution agreement so as to deprive Bill of his ability to invoke the 5th if he was subjected to deposition in a civil case (you can't invoke your right against self-incrimination if the state has agreed that you can't be incriminated).
2 -- during the depos, Bill did not invoke the 5th, and testified truthfully that he was super-rapey.
3 -- the prosecutor's office reneged on the original non-prosecution deal and used Bill's depo testimony against him to secure a conviction.
4 -- the court of appeals said "nope, a deal's a deal, and Bill detrimentally relied on that deal, so you can't use that testimony against him."
What I don't quite get is why there can't be a re-trial, just not using that testimony? I'm guessing that they are invoking the full deal of "you said you wouldn't prosecute at all, so you don't get to prosecute at all."
But, in the "not so fast" department....as I understand, they have multiple other credible accusers in OTHER jurisdictions who are NOT subject to any such non-prosecution deal. If I was Bill, I'd keep my lawyer on retainer.