To be clear, what sphagnum moss does is release some chemicals that are likely to be powerful surfactants or related compounds that disrupt bacterial biofilms. It is similar in its effects to what products such as
AquaFinesse™,
Ahh-Some®, Spa System Flush and Multizyme do (the latter uses mostly enzymes).
The main place where such products find some reasonable use is in high bather-load commercial/public pools that tend to get bacterial biofilms in their sand filters. The other place some of these products are useful is in removing existing biofilm in new or poorly maintained residential spas where wet-testing from the factory can leave some water in the pipes where bacterial biofilm can form. However, for residential spas, once the biofilm is removed, it can be kept away by proper chlorine maintenance.
In residential pools, significant biofilm problems are a rarity and can almost always be prevented by proper chlorine maintenance. Note that in residential pools, the majority of chlorine loss is from breakdown from the UV in sunlight. Actual measured overnight chlorine loss is typically very low. Any products that kill bacteria or algae or prevent biofilms are going to have a negligible and unmeasurable effect on a properly chlorinated residential pool such as those maintained by the tens of thousands of members of this forum (and the hundreds of thousands who look at the forum but do not register).
As to whether the sphagnum moss would prevent bacteria from growing to convert CYA into ammonia if a pool is "let go" with zero chlorine (say, over the winter), I do not know. Unless the chemicals in the moss cut down bacteria growth to very, very low levels, the months over the winter are a very long time for bacteria so that even a slow growth could consume voluminous amounts of CYA and turn it into ammonia. Simply inhibiting biofilms is not enough since bacterial growth outside of biofilm formation is enough for such conversion. So my hunch is that it won't help, but I can't say for sure. There are other ways of making sure you don't get a CYA-to-ammonia problem, namely maintaining a chlorine or algaecide level in the pool over the winter, closing the pool when the water is as cold as possible and opening in the spring before the water warms up significantly.