So, if some fringe group sends anonymously posts a letter claiming to be from anonymous, doesn't that make the letter from anonymous? I thought the whole point

So, if some fringe group sends anonymously posts a letter claiming to be from anonymous, doesn't that make the letter from anonymous? I thought the whole point of a group that is completely decentralized is that anybody can claim to be from anonymous and that is enough to mean you're from anonymous. I agree this group probably posted the letter as yet another way to get attention, but isn't the mere fact that the letter claimed to be from Anonymous enough to make the letter from Anonymous? How can a group without central leadership claim to not be behind an anonymous letter?