Earlier quoted context omitted.
Publish the protocol docs. That's literally all that's required from them. Actually they don't even need to - they can just promise not to sue anyone who reverse-engineers it and publishes a commercial client. That's how adversarial interoperability worked for decades (and gave free software the ability to interoperate with proprietary formats, see LibreOffice for example) before abusing the DMCA and/or threatening l…
Can you articulate where in the DMA where it says that all Apple has to do "is promise not to sue anyone" to be in compliance. Or where it talks about protocol publication. Hint: it doesn't.
The existence of competitor would by itself be enough proof to the fact that Apple is not restricting competition. But for such a competitor to exist, they would require enough assurance that the business will be viable and they won't get sued out of existence.
Apple either publishing the protocol or at the very least publishing an official licensing agreement allowing anyone to reverse-engineer and reimplement said protocol would achieve this.