Like bro, maybe people need to watch the film where the person you’re wanting to be The Joker clearly states that he was never Joker and that his name is Arthur, and then gets shanked by someone who may or may not be the actual Joker. You know, the one who wears a purple suit and fights Batman?
Arthur is not the Joker. You can be mad at Phillips for deceiving you, or WB for marketing or whatever. But in the context of the film and whatever Batman universe it’s set in, he is not the actual Joker.
It’s really not that complex of an idea/twist - and was heavily theorized even when the first film came out that Arthur was not the actual Joker.
If you’re having a hard time understanding that, then it’s on you.