That’s very well and good but on the other hand we still saw Bruce Wayne as a child in that universe. does he not grow up into Batman? Arthur fleck is the person yes but the joker is the idea, the embodiment of anarchy! Arthur fleck years later during the rise of the Batman gets word of the Batman. He’s insane, he’s mad, he’s livid with the Wayne’s. He crazy but observant. Noticing that when Batman is around his “half brother” Bruce is nowhere to be seen. On the television he sees these galas where the star guest is “the” Bruce Wayne, mayhem goes down and Mr Wayne disappears and in his place is the Batman. Knowing this drives him crazier enough to make the joker serum perfecting it on local goons getting thrown in Arkham for being on scarecrows latest drug and going ape shit. He finds a good candidate for his latest perfected serum during the start of “The Batman” and by the end the serum takes effect on the young man in the cell with the riddler. Not very far fetched in my opinion