Reviewing news coverage of the Core Technology Fee, usually glossed over is that the CTF applies to every install of a marketplace. From Apple:

Developers of alternative app marketplaces will pay the Core Technology Fee for every first annual install of their app marketplace, including installs that occur before one million.

Because marketplaces will usually be free, this makes it nearly impossible for them to work without charging developers just as much as Apple does, or having enough cash that it can lose money. Everything in Apple’s rules is designed to prevent anyone from using this.