PM-“Igros is Reb Moshe’s psakim halacha l’maaseh, and he writes that turning down a flame is NO BETTER then turning it off. If turning the fire off is already muttar, as you maintain, why bother??? It doesn’t need to be better, only the same.”
Again, Rav Moshe is saying in his t’shuva: Don’t assume that lowering down the flame is any better than shutting it off completely. Rav Moshe is saying that if you’re going to hold that it’s assur to turn the flame off, then that same logic will determine that it’s assur to lower the flame as well.
In the paragraph in question, Rav Moshe is not saying that this is his shita; only that in this scenario, the logic is flawed.
Peace brother.