Howdy, Trev.
In this particular vid it looks like plain old yard dirt with small rocks mixed in. I do the same, but use Spackle dry mix with Elmer's Wood Glue mixed in for strong adhesion. I personally think using the commercial dirt is a waste of $$, because he covered it up completely, none visible at the end. If using it for a base only, that's expensive. I do like how he's using mesh drywall tape, good idea and I'll remember to use that in the future. Using crushed clay cat litter is another good idea as well. Just add sand and small rocks to the landscape.
IMHO, most of the commercial ground work material takes a back seat to any natural material I have right outside in my garden, much cheaper, too. I hope that helps.
Cheers, Ski.