> People configure things similar to https://mxtoolbox.com/dmarc/dmarc-setup-cname instructions (which I find in conflict with RFC1034).

I don't think they're advising anyone create both a CNAME and TXT at the same label - but it certainly looks like that from the weird screenshot at step 5 (which doesn't match the text).

I think it's mistakenly a mish-mash of two different guides, one for 'how to use a CNAME to point to a third party DMARC service entirely' and one for 'how to host the DMARC record yourself' (irrespective of where the RUA goes).