Not a done deal yet. My understanding is, it's pretty much the defense announcing their intentions, but the prosecution hasn't really offered anything at this point.
My guess is with what the defense was planning as a strategy, which was Hunter was too F'd up to have intentionally defrauded the IRS, somebody in the gubment "suggested" they plea and avoid a bunch of messy headlines right before the election. If the Dems win, he'll be pardoned anyway.