Yeah, so I believe it's definitely fake. After all, it's so ridiculous, it cannot be true.
Vegapunk makes a huge message to the world and the Gorosei, having the risk to expose their demonic body to the world, are trying their best to stop the broadcast, yet the same Vegapunk apparently doesn't "know that much" and he only says Joyboy was known to have stretching powers and that Mother Flame is spelled M.O.T.H.E.R F.L.A.M.E? The last detail is irrelevant to the OP world, not to mention that many people might even be illiterate, so how does the correct spelling even matter?
I rather believe these stupid spoiler provides just wanted to bait the entire fandom, lmao.