So, as an engineer, I'm a HUGE fan of destructive testing. Finding out the failure mode of things is a critical part of building something dangerous, and HPA tanks definitely are very very dangerous. So taking a couple shot at HPA air tanks with a representative airgun (or paintball gun) isn't necessarily a crazy thing to do because, when you're on the range, accidents are very much are a possibility and one errant shot could create a hand grenade.
That said, this test was done in a pretty astonishingly unsafe manner. Aside from the distance being way too close, the cords obviously not being strong enough, a lack of a protective barrier/remote actuation, and the obvious fact that he was nearly hit by one which rolled right past him, he had no way to safety the system after the test. Yes, if you're going to do destructive/penetrative testing on a system like that it needs to be fully pressurized, however you need a way to safely and remotely de-pressurize the tanks afterward. Precisely what you should NOT do is then "walk up to it to see what happened." 75 yards with nothing in between is a pretty pitiful safety measure, but even still that is vastly better than walking right next to a violated HPA tank.
So yeah, free country and I hate to be a safety sally here, but this was a really bad plan on several different levels. Similar goes for stringing them together, because now all three are in an "unsafe" condition needing to be vented and without any sort of remote monitoring equipment you have no idea if that has occurred and the test range is safe.
I should also add that bomb ranges exist for a very important reason, and are one of just a handful of appropriate places to test such a thing.