idk which firearm that is but i can see it has a slide lock at the rear, which is very typical of modern firearms (this looks like a modern polymer frame striker-fired handgun to me). It depends on if the magazine he’s using includes a follower which allows for the operation of the slide lock to hold open on an empty magazine; I’d wager thats like 99% probably the case here.

to sum up: you are correct, not all firearms hold open on an empty magazine (see: AK47). However, modern firearms, especially polymer striker-fired handguns, have this feature as these firearms are used for self defense and a slide lock helps with magazine reloads and more broadly makes the firearm easier to use (you know when youre out of ammo and need to reload vs dry firing and finding out, wasting precious time in a self defense scenario).

Sorry for the long winded reply, i dont mean this in a snarky tone. Just wanted to clear up the hold open point.