ChatGPT is, to my knowledge, trained on Reddit and at least certain sub-reddits are basically people (or bots) telling others that they probably have ADHD/ADD. These are the "AskReddit" type of sub-reddit. There's a Danish subreddit for everyday questions (advise column style posts), and like 80% of people there are apparently either autistic or have ADHD.

So I'm not entirely surprised that an LLM would start assuming that the user have ADD, because that's what part of it's training data suggests it should.