Well, I can say that all the people shooting IPSC who shoot Glocks use this technique to save time.
The ones who aren't very good might. If you really want to save time, you don't shoot the gun to slide lock.
One of three things, or more likely a combination, could be happening to cause this technique to work:
1. Because the grip is angled, slamming the mag home causes the whole gun to move forward (toward the muzzle) as well as up. The slide, due to inertia, wants to stay to the rear. The frame moves forward in relation to the slide, which causes the slide stop to disengage the same as if you pulled the slide back. (The Glock's steeper grip angle and lightweight frame would explain why this seems more common in Glocks.)
2. If there is vertical play between the slide and frame, slamming in the mag causes the frame to move up in relation to the slide and slide stop. As the frame hits the slide, the slide rebounds and moves up in relation to the frame, but the lighter slide stop does not rebound, and drops out of engagement.
3. Similar to 2, but instead the top round in the mag pushes the slide up away from the frame farther or faster than the (likely only partially engaged) slide stop can follow.
A weak magazine spring or worn follower can contribute by only partially engaging the slide stop.
Any of these could eventually cause the slide stop and / or the engagement notch in the slide to wear and round off to the point that the slide no longer locks back on an empty mag. (but no faster than manually thumbing the slide stop to release the slide causes the same wear.)
In any case, the "slam reload" is essentially an induced malfunction, and should not be relied on as a technique. If you want speed, seat the mag and thumb the slide release as you re-aquire your weak hand grip and extend back into your firing stance. If you want consistency of technique, seat the mag and rack the slide the same as you would a tap-rack.