Roger said
At high angles of attack, it can get far enough
back that air does flow forward across the wing.

indicating that is what lifts the stall warning tab.

That's pretty deep into a stall. In airplanes I've flown the tab lifts
at high angles of attack, but I doubt very much the wing is very much
stalled. The horn sounds well ahead, and at lower angles of attack, a
stalled wing. For sure, though, the stagnation line had moved below the
tab, lifting it.