They seemed to be firing it with successive charges 150 grains higher than the last, from about 300 to the final >1000+. I guess they wanted to "prove" how tough these "non-firing" replicas are. They are possibly built well enough to handle normal charges, but I have no way of knowing that. The idea of buying one and drilling the vent hole for actual firing use scares me enough not to do so.
In the video after splitting the muzzle (but before discovering damage mid barrel) he says it could probably be cut off and still used. I wouldn't do that even with a certified, proof-tested, intended to be fired model once it had been subjected to overcharges like that.
If I was going to buy a replica it would be one made for firing and I'd stay within the recommended range of charges/loads, whatever they might be. That said, my only experience with muzzle-loading long guns is a Hawken style 50 cal that I bought from Buds a few years ago. It was kinda fun and made lots of cool smoke.