If the average user isn’t supposed to do it, it’s fine. I already side load Fortnite with my developer account, but would definitely make it a lot easier through Epic Games StoreAs mentioned here: https://developer.apple.com/news/?id=qs5bol0g
View attachment 2441130
So this is actually supposed to work, although it seems to be a bug that it doesn't yet.
Or maybe developers can only install their own marketplaces (signed with their own developer certificate)? That's not clear. It's also not clear if this is supposed to continue working after moving to a stable, non-beta release. Or maybe it works for all developers with a paid developer account. Too little information around, but for sure something is going on.