Yeah it was pretty clear Matt Reeves read The Long Halloween and part of Zero Year (but got filtered by the Red Hood) and came up with a one and done story, he was probably not expecting to make a sequel much like Joker.
Expect Batman II to be Folie A Deux levels of fumble for this reason, you'll wish Gunn had the balls to take away both Arthur Fleck and Battinson away and force them into the Extended Universe.