I don't have a reason to think it is a myth, but I wouldn't recommend a generalization about any ""type" of coral.
Many soft corals do well in elevated nutrients. Some also grow even in ULNS systems.
Do they grow faster with more nutrients? Depends on what you are comparing to what. I've not seen a study of the growth of soft corals in different nutrient levels (scientists do not seem to like to study soft corals as much as hard corals), but I think many reefers have found that many soft corals grow rapidly in elevated nutrient systems.