It is not the scope of DMA to identify and regulate an OS, or demand a hardware-vendor to open and document his OS at his own expense.

They identified something more abstract and much less arguable:

They identified that Apple has created a closed market of significant size, made themselves the gatekeeper and invited companies to compete there.

But ON TOP Apple decided to enter the market also as a player, and skew the playing field in their favor.

So it's an unjust market where forces are unable to flow freely, and the EU is attempting to rectify that.