That was my thought also. If anything, sling shotting the slide would seem to cause greater slide velocity given that the slide will travel a longer distance than the release from slidelock.
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Some class or other I attended it was noted that Glocks have slide stops, not slide releases; that is, the slide stop is so small because it is not intended to be used as a slide release. YMMV. IANAL. You be you. And Fight The Derp!
That said, which of the three methods of releasing the slide (overhand, slingshot, or slide stop) is best is one of those perennial topics along with 9mm v .45, 1911 v. Glock, and whether the FAST should be used as a drill.
Derp a derp a derp a derp a derp...
Rumors to the contrary, the slide release works very well on a Glock. If the standard OEM part is too smoll for someone to run it with their thumb, the factory and aftermarket have you covered. In fact, the new hotness Glocks come with ambi slide releases.
Do what makes you comfortable and happy, I’ll keep on dropping the slide the fast, efficient way.
I would remove that person from my list of credible sources.
David