He's not wrong...
Actually, what the instructor said was correct for a glider because you fly the circuit SLOWER than best glide speed.
If you are too high, you should pull back/reduce speed and this increases your glide angle (because you are slower than best glide). You don't want to push the nose down if you are high, because you would then come in too fast. The glider may not have enough drag (even with spoilers) to dissipate the extra energy and as soon as you hit ground effect you'd just keep going off the end of the airfield!
So, if you pull back to avoid landing long, you push to avoid landing short. But you are pushing to increase speed to your max glide speed (plus a bit for a headwind of course), not to dive in at a faster speed.
The second possiblity is the idea of clearing a fence by getting into ground effect early with some extra speed. Get within half a wingspan of the ground and use the extra kinetic energy to maintain that height. Drag is much reduced in ground effect, far more obviousy than in a power 'plane, mostly due to the longer wingspan. This only works if the area in front of the airfield is flat & clear of course.
Having said all of that, I don't think it applies to a dead stick landing in a power airplane because you would already be flying at your best glide speed, so pushing or pulling will increase your glide angle.