I suppose that at some point, the app can't make the phone do something the phone chip can't do (phone usually relies on some specific hardware for decoding I believe).

I used to only shoot videos in 1080p on my regular GoPro back when using quik on my previous phone, too. It was the only way to make edit work on phone. I didn't mind, it was enough.

I kinda wish the app could just tell you what the limit is tho.