I think the reason some people might believe this claim is because we’re taught in school that the moon’s gravity causes the tides. I think the reasoning goes, “well if the moon’s gravity can affect the tides, surely it can affect smaller things too”
It weighs the same, clearly the moons surface is ionizing reflected light in a way that triggers the reaction.
You’re forgetting the sun’s gravity. Moon phases are caused by the relative angles of sun/moon/earth. So are the various levels of tides. It must affect werewolves as well.
Not as much as vampires.
This is why they only change during a full moon; less moonlight than that is not enough to catalyze the reaction.