Humic acid is not a single substance, but a broad mix of organic components.
Even at 100°C, some of it is breaking down. About half is gone at 300°C. By 400°C, almost all of it is gone.
As far as I know, ADA has never revealed to what temperature their substrate is heated.
But I'd consider the breakdown rate a good a clue. If I had to compare the reported breakdown rate and softness of AS to a known, it would be kitty litter, which is fired to 1,000°C. Or maybe akadama, 800°C.
I know of no other method than heat to slow the breakdown rate of clay to the lifetime AS achieves. And though I hate to bring it up, there's a certain amount of "magic" I've observed surrounding the ADA products and culture, rather than just science.
So, while I am certainly making a few assumptions, I think this is probably one of those times when "organic" is being used to appeal to a certain group of people, rather than truly descriptive of the substrate. While the ingredients that were put in were organic, what comes out of the oven may not be so.
Life happens. On semi-hiatus, checking in infrequently.