I've done similar things with perlite, I believe. I would not call the inert material in what you call the "rez effect tech" substrate, as it serves a qualitatively different purpose.

The key difference between the method you mention and what appears to be the case here, is that you don't treat it like bulk sub, afaik, you essentially crumble and case. This allows the mycelium to re-knit throughout the nutrient-laden brf or grain layer and then grow up through the inert moisture-containing layer. Imo, this is significantly different than creating a homogenized mixture of spawn and inert materal, as appears to be the case in this thread.