Alan Hodgson:
I really believe that's incorrect. Relaxed alignment specifically means you cansign with a subdomain's key or use a subdomain for SPF. Read sections 3.1.2 and 10.4 of that same document, for instance.
Hm. https://tools.ietf.org/html/rfc7489#section-10.4 reads like you're not wrong.
I'll ask some dmarc professionals for clarification ... Andreas