From the reports I've read, it looks like they're going to either plead no contest an "Alford plea" (effectively, a guilty plea in which the defendant does not actually admit guilt).
I don't know if these guys are actually guilty - the evidence seems inconclusive, at best - but if I were locked up for 20 years of a life sentence and the prosecutor offered to release me with time served if I plead guilty (with or without admitting guilt), that would be a tough deal to turn down, regardless of guilt.
It’s all a dog and pony show to save face for the prosecutor. Everybody knows they botched this case, and the prosecution is basically admitting it without having to admit it by coercing the plea with the promise of freedom.