as i read this, skipping so very much of it.
the salesman from the LFS the OP was talking about, ...
ammonia released from decay, ... ya, this is true, unfortunately plants don't have a preference for ammonia, and thankfully there is beneficial bacteria that do, and they have a tendency to convert to nitrites, and the next stage in the (overly simplified cycle) is bacteria that convert nitrites to nitrates, ... and this the plants accept eagerly.
it's about the easiest source of nitrogen in our tanks for plants to consume
i don't remember for sure, but i think ammonium is the preferred choice for plants for a source of nitrogen, followed by nitrates. ... i could be wrong about the order, it's been awhile.