I had numerous colonies STN/RTN in my tank over a year ago. At the same time my alkalinity, nitrates, and phosphates were all over the place. Once I got them under control the STN/RTN stopped. I have a colonies halfway gone that survived. Are these protozoans you mentioned sensitive to the parameters of the tank? Interesting hypothesis, I wish it was true, but I find it difficult to believe that they are the only reason for RTN.
I think that when the parameters/problems are corrected the coral can somehow fight off the parasites/infection.
Maybe the Philaster don’t sense the corals weakness anymore and it turns them off or maybe the coral gets its protective slime coat back which slows or stops the infection. Just thoughts!