“Obama was holding the possibility of military proceedings over Petraeus’ head.”
He’s retired from the military.
In the recent past, Generals have been recalled from retirement and put back on Active Duty in order to be investigated and punished for actions they took before they retired. Petraeus knows this. Petraeus knows it can be done to him. Obama could keep that from happening, though.
Not that they will, but they can and have done so with others- the military can yank him back under their jurisdiction and bring action against him for any violations he is believed to have committed before he retired...including adultery.
Technically he does still have the possibility of facing military proceedings.
As a retired officer, Petraeus may be recalled to active duty up to his dying day.