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”
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.