The law talks about "a dominant position within the internal market or in a substantial part of it". If we look at the iOS platform, Apple could be seen to have a dominant position there since it is the market maker there.

The EU appears to have specific criteria for when a single company's product can be considered its own market. [1]

Dominance on the aftermarket is excluded to the extent that a customer:

1) can make an informed choice including lifecycle pricing, that he 2) is likely to make such an informed choice accordingly, and that 3) in case of an apparent policy of exploitation being pursued in one specific aftermarket, a sufficient number of customers would adapt their purchasing behaviour at the level of the primary market 4) within a reasonable time.

The case that established this precedent found that printer manufacturers did not have dominance in the market of "printer cartridges compatible with their printers" based on the above criteria. Whether or not this would apply to the iOS platform will have to be decided by a court.

[1] https://europeanlawblog.eu/2013/09/26/the-efim-case-no-domin...