I already contacted the DMA authorities and complained how this has an effect on German diabetes communities and they replied that I am not the first one who approaches them on this and they are already investigating it.

Google is just trying how far they can push this.

https://www.eu-digital-markets-act.com/Digital_Markets_Act_A... Art 6 (4). Read it to the end. That's how.

The same way Apple is allowed to do it presumably.

This is arguably required by Article 30 of the EU Digital Services Act.