I wonder what's fueling all this FUD about Biden not running.

I think it's just attempts to destabilize Biden as a candidate. Saying Biden should drop out, and that he's not making his own decisions, are just attempts to weaken his candidacy. Along with reminding people of other younger candidates, they say he should drop out ( totally unrealistic), raise the specter of Harris becoming President (step down scenario or incapacity), never mentioning both of those scenarios apply to Trump. There will be a steady barrage of this (trial balloons) as the opposition searches for the right combination to weaken Biden. They will save an October surprise to hopefully nudge the undecided away from Biden and to Trump. Comey's letter was unscripted (we think) but was very effective at derailing Hilary and pushing Trump over the edge. Comey helped put Trump across the finish line but was let go because he flunked Trump's loyalty test.