The DMA applies to both Apple and Google (and others in related areas). Whether 30% of smartphones are iOS and 70% are Android, or the other way round, is irrelevant. In either scenario, both companies are in a position to monopolise their platforms, by being the only source of software.
Your argument hinges on the fact that Google don't currently choose to block app stores other than their own. But nothing's stopping them - at least legally. If they were to do so, we really would just have the choice between two monopolies, and switching platforms would be pointless.
The DMA is a legal measure to ensure that doesn't happen (amongst other things). And not just relying on Google's largesse, whilst letting Apple do what it wants.