Here, let me give you some ammo!

blanks?

Could be

Blanks still flash. That's basically the whole point of blanks.

Not enough for the camera’s frame rate to pick up.

I don't know what type of camera they are using nor if it is a blank, what type or how "hot" the load is. So it is theoretically possible that the flash wouldn't be detectable here. But- if this is a blank, and they really did this- why is no shell ejected when the action cycles back? Any cartridge that went in, even an electric blank would be ejected when the slide cycled and the ejector passed over the shell. Who knows. Maybe it also doesn't show in the video, and maybe the fact there is no smoke visible, no flash, and zooming in on the ejector port after the cartridge is "thrown in" shows no sign of a cartridge in the chamber? Speculatively that cartridge looks rather large. Maybe perspective but looks to big to be a 9mm or smaller cal. Round. The recoil of the weapon seems little more than that of the weight of the slide. Or maybe it was a very weak blank, or he's very good at handling recoil from a .44+. Who knows?