The law will require to allow third-party app stores. If Apple decides to impose 27% tax on everything sold in the third-party app store then I'm sure other EU competition laws will kick in. As Apple will be explicitly using their power to make direct competition non-viable.

Also, EU does not need to spell everything explicitly. The law will give the EU Commission a lot of executive power to respond to this type of shenanigans from gatekeepers.

Article 7, compliance with obligations for gatekeepers, paragraph 2:

Where the Commission finds that the measures that the gatekeeper intends to implement pursuant to paragraph 1, or has implemented, do not ensure effective compliance with the relevant obligations laid down in Article 6, it may by decision specify the measures that the gatekeeper concerned shall implement. (...)

Article 10, updating obligations for gatekeepers, paragraph 1:

The Commission is empowered to adopt delegated acts in accordance with Article 34 to update the obligations laid down in Articles 5 and 6 where, based on a market investigation pursuant to Article 17, it has identified the need for new obligations addressing practices that limit the contestability of core platform services or are unfair in the same way as the practices addressed by the obligations laid down in Articles 5 and 6.