Apple, Microsoft and other companies have faced EU antitrust/anticompetitive legislation scrutiny for years. None of the relevant companies (those having market power) are getting a pass here if they are violating any of the regulations.
Apple has been accused and investigated by the EU for having a "monopoly" before, a fairly recent example:
https://www.cnbc.com/2021/04/30/eu-says-apples-app-store-breaches-competition-rules.html
I think the DMA legislation is way to try to help make EU antitrust laws and regulations more clear and defined. Do I agree with everything about it? Not necessarily, but I do happen to feel that Android and iOS have dominant positions in mobile OS and neither should be able to restrict alternative app stores, alternative browser engines, etc.
Imagine if there were only two phone carriers in a particular country/region and each of them only allowed one brand of phone e.g., one only allowed Nokia phones and the other only allowed Motorola phones. Apple, Samsung, Google, etc. wouldn't have had a chance in that market. I would be opposed to that just as I am similarly opposed to Android and iOS "access" restrictions. One of the roles of antitrust laws and regulations is to prevent scenarios like these from happening when there are only two or so dominant companies in a market.