Pardon my French, but where the primer fell into.. ?
Watched through the whole video but can someone draw a rough picture or some thing splaining.

Right here... the screw didn't protrude far enough into the receiver and left a pocket for the primer to fall into. So the story goes.
I have a hard time visualizing how that could happen. The case would be held against the bolt face by the extractor, leaving the primer with no place to go until the bolt was far enough to the rear for the ejector to do it's work. By that time the case should have been well over the magazine.