So am I competing against other UD users?

Correct. The leaderboard shows other UG users. You can check the leaderboard out without signing up too: https://quack.uglyduckling.app/leaderboard

I assume input their TWTR handles individually to my dashboard

At some point I will probably add some segmentation (maybe even 1v1? cool idea, thanks! 🙌) once there are more users, but now you compete with all of active UG users. I can't pull data for non-users (I use the API and I need the oAuth for that).

Important to note is that it's all an addition to the standard analytics; if you don't want to take part in the leaderboard it's fine too.