This is the product where they mix elemental S with compost? The theory makes perfect sense to me, the biology in the compost will be digesting the S in the mixture. Somewhere in there will be biological strains that flourish in a high S environment, so when you spread the product on the field those microbes are going to work in the soil doing the same thing. The other big benefit, is all the secondary metabolites the microbes produce.
I know nothing about this product, I would be curious to see a nutrient analysis verses what is claimed to be in it. Once an element has been through a microbe it gets converted into an organic form and can't be detected by mineral analysis. My guess is that an analysis will show lower than the claim, but the product will perform better than said content of S, due to the fact it is now in a plant available form.
My biggest concern would be if the compost was made properly, so the right population of good microbes was fostered.