> If the Core Technology Fee does not apply to Your Applications under this Section 4.4, any Alternative App Marketplace (EU) You distribute may only distribute Applications from You or another developer registered with the Apple Developer Program and not subject to the Core Technology Fee under this Section 4.4.

So a hypothetical F-Droid for iOS would need all its apps to be from registered developers with fee waivers. (And individuals are not eligible for the waiver, so it could only contain apps from other non-profit organizations.)