Apple will be required to "allow and technically enable the installation and effective use of third-party software applications".
IANAL, but I doubt a policy of "sideloading is allowed, but only for apps that we approve and/or developers who pay us" would be considered compliant with that.
They could enforce some sort of signature requirement, but if they tried to force developers into some sort of business relationship before being able to sign, that seems like failing to allow third-party software.
It's almost guaranteed apps will still have to be signed by Apple in order to be sideloaded. Current developer sideloading requires signing.