You’re absolutely right, that is a very tight pattern....squib maybe?
I don't know, that's what I am thinking. either way, that load was still mostly in the wad when it struck... that means real close range. I don't care what kind of magic polycarbonate layered voodoo material it was, it wouldn't survive that blast unless something else was going on. That load was traveling very slow when it hit. Maybe it was someones custom "cut" shell or some other baloney and only a partial ignition due to moisture. Too bad we can't find out all the details, I'm pretty interested in knowing how this happened. I'm not doubting it really happened, but I do know this wasn't a normal shotgun shell.
I've got a lot of years experience with shot loads. I could see the material possibly surviving like this past 30-40ft or something, but there is no way to keep a pattern that tight at 30ft or beyond... even 30ft is a stretch of sci-fi thinking. Add to that the choke is gone off this shotgun, it's a lead vaporizer at this point.