The Android TV user interface is locked at 1080p but the actual video content plays back at 4K. On Disney+ if it says Ultra HD or Dolby Vision then you're getting 4K. The 1080p user interface is just a scaled overlay, it doesn't prevent 4K video playback.
You can't test this with Disney+ but you can with YouTube using their Stats for Nerds feature. When playing back a 4K video it will show a viewport resolution of 1920x1080 but a current/optimal resolution of 3840x2160. The viewport resolution is the GUI and the current resolution is the video playback resolution. It works the same way for Netflix, Prime Video, Disney+, HBO Max, Kodi or any other video app. The interface is limited to 1080p but the video signal isn't.
tl;dr - There is nothing wrong with Disney+ and 4K works just fine.