It's actully a 15 year old law (in it's current name, originally from 1958). From the article:

> Today's decision concludes that Apple's anti-steering provisions amount to unfair trading conditions, in breach of Article 102(a) of the Treaty on the Functioning of the European Union (‘TFEU')

These fines aren't through the new DMA. These fines are through preexisting regular antitrust laws.

The specific guidelines that the Commission is relying on were written in 2006 [1].

[1] https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX%3A...

This argumentation is probably being based on generic anti trust law ... and the DMA is a new specialization below it. Does not mean that you violated past law just because new law is also applicable to you.