Try dropping flap while in the landing flare
Errr....What this gentleman seems to be suggesting is that you go out in an aircraft and try something you have never been taught and which doesn't appear in any manual I've ever seen.
If you are doing it in someone Else's aircraft I suggest you get their permission first, before promoting yourself to test pilot!!!
As to the original question, the way to "Stretch" a glide is to fly at the best L/D speed for the configuration you are in. If you are at, say, flap 30 and flying faster than the best glide speed for that setting, retracting flap will,in fact, reduce your glide angle.There will be a significant trim change (usually nose down) and a need to fly a different attitude.
If you are AT the best glide speed for flap 30, retracting flap will put you BELOW the best L/D for the new setting. You would need to speed up (i.e dive) to get to the new best L/D speed, eating up height, so if you are already low it's not necessarily a good idea.
No easy answer, I'm afraid, but,as always, it depends upon the aircraft and situation you are in at the time.