I documented the problems their app was causing, sent the documentation to the offending app. They promised to fix it in their next update. In the mean time, I have deleted their app off my machine and the bugs caused by their incorrect UTI declarations are gone. This doesn't answer the question of how to correct the mdls entry, but it is a workaround.

The bugs preventing UIDocument-based UIScene-based MacCatalyst apps from working properly have been fixed in the macOS 12 betas a few weeks ago. Still don't work right in macOS 11.6.

This is fixed in macOS 11. (tested in 11.5.1)