At the end of the day, you have to pay $99/year to publish on the Apple App Store, and on Android, although Google has had some shady practices around automatically flagging/removing apps from it's store, you have more options in terms of app distribution, such as F-Droid.

To my (now deleted) comment's point though, I don't see much of a relation between F-Droid and NewPipe, besides that F-Droid hosts it.

EDIT: other comment on this thread answers that last point I made and your question, and sums up what NewPipe has to do with the "openness" of the platforms [1]. However, I now wonder, what is stopping Google/YouTube from going after NewPipe & F-Droid?

1: https://news.ycombinator.com/item?id=23874943

Or, an app that would go as far as doing such deviant things as providing its own custom web engine and allow an iPad 2 to remain relevant and useful in 2020.