Sabba: You’re the only one here saying the the accuracy of the claim is irrelevant. Truth tends to be a defense against defamation. As for lashon hara, as Gaon said, its very important for people to know whether their relying on a psak from Reb Moshe or from a Rabbi who would commit fraud.
To respond to your straw man argument of “If someone were to say that Rav Moshe Feinstein zt”l copied a teshuva from someone else etc.” I will use another straw man as I like to do:
If I were to write my own halachah that it’s a mitzvah to give me 20$ a day, and I appended it on the end of Iggeros Chazon Ish, would we have to assume the Chazon Ish actually said it so as to not talk lashon hara about me?