I just tried it with a Glock 43. With the mag full there's a lot spring pressure holding the first round against the bottom of the slide. I can't get the slide to move more then an 1/8th of inch. With it empty, I still can't snap the slide all the way back. It'll move pretty good but not all the way.
There are several different scenarios where the slide could be closed after running dry. There is a guy on Instagram called @milspec_mojo. He can do the same thing. His has a upgraded recoil spring and quite a few thousand rounds through the glock. Me personally I'll rack it off my belt cause I wear one built for that or off something else. Benefit of having upgraded steel sights on a glock with a good shelf on rear sights. It really depends how you train and what is fastest with the least amount of steps involved. Cause that's what is going to matter when shit goes down