Issues with attention are one of the smallest problems of adhd. It's only named after that because that's what parents and teachers notice on kids and get annoyed by. The same way as you get to be labeled a "high functioning" autistic if you learn how to not annoy others with your problems.

The world disappearing when you're doing something you care about feels very adhd tbh. The underlying issue with adhd is that our brain often fails to get positive feedback from doing small things like completing tasks, then over time the brain starts resenting having to do these things because there's no reward, and it tries to force you into looking for something more rewarding to do. When It does find something rewarding to focus on, it latches into it and doesn't let go.

Me, personally, I feel like I have two moods only: distracted and annoyed. If my focus is on something rewarding I'm distracted by it and think about nothing else. I forget about troubles and tasks and responsibilities. I feel rested and happy. When that's not the case, I get so annoyed that just existing feels painful.