That's a P320 Pro-cut slide. It's the subcompact length, he probably took the 3.6" barrel down to 3.5

Definitely not a P365 slide, which would not fit on the P320 FCU.

Totally asinine.

In the comments section of his Youtube page the guy pretty much admits people were pulling his leg and he fell for it.

And yes, a 365 upper will not fit on a 320.

Running a shorter slide on a longer frame or larger frame isn't new. Parker Mountain Machine and Grayguns both have compensators available (Grayguns P365, and PMM, P365 and P320). These can extend the slide beyond the frame dust cover, or with the use of a larger frame, can run a short slide with a comp on the end.

The pistol in the video amounts two just the shorter slide with no compensator.

I plan to set up a P320 that way, using a compact slide on a full size frame, with a compensator on the end, on a tungsten TXG grip module.

With just the short slide on the full size frame, with the dust cover sticking out beyond the slide and barrel, it does look a bit ridiculous, but I suspect the user wasn't running it for looks.

I'll say it again.
Totally asinine.