Yes, I'm well aware of the capabilities of the AR lower, and indeed have seen them made from everything from plastic cutting boards melted together to 3D printed.
I guess my point was that it was more... clickbaity, than truly educational. There's thousands of videos showing people making lowers (as noted above) from all sorts of materials, and a thousand more showing people making furnaces and casting aluminum.
His merely combined the two, which made for a fairly interesting video, but really, taught us nothing at all new, except maybe for bad gun handling.
Honestly, all this guy did was cast a block of bar stock that had three pre-drilled holes in it.
What I'd much rather see is somebody taking an existing lower, waxing and coring it, using IT to ram the molds, and make a much more dimensionally-close casting.
Or 3D print a 5% larger master out of a burnable filament, coat it with slurry, melt it out and then cast it. With a bit of practice, I suspect you could get close to an 85% lower.
(No, wouldn't be as cheap as factory lowers or even off-the-shelf 85% lowers are today, but then, this guy wasn't exactly doing it to make money or do it cheaper than store-bought either. )