Correct me if I’m wrong but aren’t most separated (whether just didn’t re-enlist or retired) military part of the “inactive ready reserve” for some number of years? As such could they recall you to active duty and court-martial you if you disparage the POTUS?
No.
Folks do stay in the IRR for a while after separating, but IRR members aren't subject to UCMJ unless actually on orders. Even if they could convince someone to cut recall orders for this, they couldn't hem you up for stuff said while a civilian.
I suspect the worst ONI could do to a retiree is yank a clearance, making those sweet contractor gigs off limits.