This isn't just AI. AI doesn't care about jokes or memes or "professionalism." This was either a review by an actual human that didn't realize people are born on April 20th or an AI told to reject resumes with that date in it.
Either way, it's a really dumb person that set this rule.
My guess is it's programmed to reject 4/20 anywhere it finds it and the programmer didn't take into account that some values can just happen to be 4/20.
I interviewed a guy whose dev career involved a (pretty good) front-end for a grow operation. I believe the ownership at that company may have opted against hiring because of that. I'll admit, I wasn't wowed by his skills enough to go to bat for him.
Hmm, might be a list of no go words they complied from somewhere. You're right, it's very unlikely that someone actively excluded 420 but maybe they downloaded a list of "bad words".