I agree with you, and I also think that their interpretation of example 6.2.1 in the RFC is somewhat nonsensical. It states that “The difference in ordering of the RRs in the answer section is not significant.” But from the RFC, very clearly this comment is relevant only to that particular example; it is comparing two responses and saying that in this case, the different ordering has no semantic effect.

And perhaps this is somewhat pedantic, but they also write that “RFC 1034 section 3.6 defines Resource Record Sets (RRsets) as collections of records with the same name, type, and class.” But looking at the RFC, it never defines such a term; it does say that within a “set” of RRs “associated with a particular name” the order doesn’t matter. But even if the RFC had said “associated with a particular combination of name, type, and class”, I don’t see how that could have introduced ambiguity. It specifies an exception to a general rule, so obviously if the exception doesn’t apply, then the general rule must be followed.

Anyway, Cloudflare probably know their DNS better than I do, but I did not find the article especially persuasive; I think the ambiguity is actually just a misreading, and that the RFC does require a particular ordering of CNAME records.

(ETA:) Although admittedly, while the RFC does say that CNAMEs must come before As in the answer, I don’t necessarily see any clear rule about how CNAME chains must be ordered; the RFC just says “Domain names in RRs which point at another name should always point at the primary name and not the alias ... Of course, by the robustness principle, domain software should not fail when presented with CNAME chains or loops; CNAME chains should be followed”. So actually I guess I do agree that there is some ambiguity about the responses containing CNAME chains.