So just thinking about it still, I suppose if he brought the throttle back slow enough he could have gotten the engine RPM to drop below the anti-feather lock engage speed before he passed the feather detent...
but according to piper when you do the feather test/lift rod test, you should quickly move the prop control back... to me that would mean you get the selector in the feather position before the engine has a chance to slow down like it would if you very slowly brought the blue knob back.