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.