TBH my reaction when he called Francis "Frank" was just to laugh, not to conclude anything specific/dogmatic
Right. I know he's not a sedevacantist, and I know that many sedevacantists call him "Frank" ... so I was merely amused by that expression coming from his lips, precisely because it was incongruous. I would not have batted an eye at Bishop Sanborn calling him that.
Either +Williamson had been reading some sedevacantist stuff or else he just slurred his words a bit, thinking ahead and conflating two sounds.