It's hard to tell what's actually happening in that clip. To me it looks like the chain is actually fully off the rear sprocket just before the rear locks up (it appears to have come off around 1:12). At that point in time the rear wheel should just be free spinning. It also appears to be free spinning during the highside, which is strange because I would expect the most likely non-rider-error issue in this case to be the chain binding between the rear sprocket and the swing arm.

I'm almost inclined to call that one rider error. Their general body language and control of the bike don't suggest to me that they're an experienced rider, and it almost looks like they've just panic-braked and locked up the rear, except it's an odd speed and place to have done that too.

That is exactly what happened. You can see the rear lock and leave a black stripe.