Apple is free to do as many interoperable components or integrated products as they please. The DMA doesn't define ANY such restriction.

What they CAN'T do is maintain an environment where their products cannot be met with fair competition by other players, by intentionally giving advantages in the ecosystem only to their own brands.

Apple might not dominate the market for fitness trackers above 150USD today if they wouldn't have prevented others to achieve the same interoperability with their iOS ecosystem.

Apples featureset for wellness tracking was not competitive, neither in function nor in price. Fitbit and Garmin were better in doing that task, but they were not able to display message notifications, apps, etc. because the required interfaces in iOS were only available to Apple's Watch.

Maybe Apple would have beaten them in 2nd Gen, maybe competitors would have followed with equally tight iOS-interoperability in 2nd Gen.

Maybe Apple Watch would nonetheless be the leading Smartwatch in the market today. Or maybe it would be e.g. the Moto360 (google it) just due to Apple's "virtue of sucking" and insistence of doing rectangular watches.

We don't know, because none of the other players are able to compete on fair terms with Apple in this segment until today. And today Apple has such a leap-start that it's questionable whether this can still be rectified.