How can they? If you can sideload they have no way to enforce this.
Side loading will likely still require a business agreement with Apple and “App Review” to get a cert that allows your app to run.
However it is indeed possible that Apple would weasel such a requirement through the loopholes, yes.
At some point Apple might see itself being split apart because of its constant and well documented abuse of it's status in the market.
This view of Apple as a pure-platform company does not comport with reality
There is no weaseling, it is very simple: the commission covers the licensing. The alternative is saying that apple must develop software, give it away for free, then let random stores make a profit selling software that they contributed nothing to.
It is exactly equivalent to the EU saying that a company should be free to use GPL software however they like, including selling at a profit, without providing the sources.