There is a universe where consumers have a right to purchase products without the market interference of exclusivity agreements, and that universe contains the EU:
> (151) In an exclusive distribution agreement the supplier agrees to sell his products only to one distributor for resale in a particular territory.
The rules are complicated and depend on the supplier's and buyer's market share, but you can read the details on page 46 of their "Guidelines on Vertical Restraints".[0]
I'm not suggesting that a company should be required to take a positive action to fulfil Apple's requirements, but if a company is producing a file which can be installed via an app store, they should not try using copyright or contract law to prevent other app stores from also selling/distributing their app.
(Admittedly there would have to be some amount of paperwork for allowing the various app stores to pay money into the app developer's account, but that could be done on Apple's side so that the developer continues to get a payment each month, with some stats on a dashboard somewhere showing which app stores the app was sold through).
[0] https://ec.europa.eu/competition/antitrust/legislation/guide...