If they’re actually serious, they might be saying it to inure you to the future bad behavior they wish to exhibit.

Yeah, this is something I notice a lot. People will claim "I am x" and use it to avoid accountability when confronted later about their actions. the X doesn't really matter, it can be a mental illness, a disorder, a belief, etc.

My favorite part is when they get offended that you are 'judgemental' for not wanting to interact with them once they make this declaration.