The conversion of organic matter to humus is a biological process, not just chemical, and if a strong base can speed that up somehow, it would be news to me. Not that it can't happen, I just don't know how. It is not the first time I have read accounts of ash + leaves making nice compost.
It would make perfect sense that ammonium nitrate would speed things up since leaves are a brown, low in N. Feed it N and decomp speeds up. Ash has no N. If it speeds things up it is through a different mechanism.
I can imagine some mechanisms. Strong base might chemically break down cell walls and begin to hydrolyze carbohydrates, proteins etc., making them more available to microbes. I would have assumed that very high pH would hinder microbial growth, so the benefit is not worth the cost, but perhaps the alkali reacts away rather soon and the microbes can then take off on the partially digested stuff as the pH returns toward neutral.
Or, it could have to do with other nutrients besides N. Such as P or K. P is a 'limiting reagent' in the biosphere, and a slug of it could conceivably boost microbial growth.
I do have huge leaf bins, and since they are oak and hickory mostly, they are going very slow. I burn wood and have plenty of ash, so perhaps I will try this in a controlled fashion on two identical piles next year.