There is a lot I don't like about Brave, but I can't fault them for having the best mobile app on market (for me).
Why not Firefox? Bad redesign release a few years back and now discernible way to change the accessibility so it holds always. Old.reddit text is too small to be read no matter what I've tried.
None of the other browsers on Android I've found to be worth the time.
A lot, Firefox ( and every other browser on iOS) are effectively skins on Safari.
Apple will only allow third party browsers that use WebKit on the App Store.
Firefox (and other browsers) on iOS are pretty limited in what they are allowed to do. It is quite likely Admiral is able to detect that easily compared to more robust solutions on Android.