So I watched the movie and I’m all confused about it. Thomas Wayne was around when Joker was created? And he is Arthur’s father? Ok what? Little help here.
The Joker movie is set in its own continuity; liberties are taken. Meanwhile, details are left vague intentionally for the allowance of multiple interpretations. While it’s almost certain that Thomas had an illicit relationship with Arthur’s mother, the baby might not have been his. (But then, he still might be.)