The discussion you refer to describes CNAME-ing a TLD (top-level domain) to another domain. That is not a good idea.
But CNAME-ing a subdomain over to a third party is actually preferred setup to arrange, for example, bounce-handling by a third party (marketing tools and such).
For example for sending newsletters via the subdomain of
newsletters.example.com by a marketing tool hosted at
newsmailers.org, you can use a CNAME record that points
example_com.newsmailers.org, actively delegating the responsibility for SPF and MX records for the
newsletters.example.com subdomain to the external party.
You cannot, however, add additional RRs (TXT / MX / A) with the same name to DNS, according to RFC 1912.
RFC 1034 section 3.6 states that the RDATA for a CNAME contains a domain name (typically where an SPF TXT record lives), while the RDATA for an MX or NS record should contain a Host Name.
Also, be carefull with CNAMEs that point to other CNAMEs. This will (most often) fail. See RFC 1912 section 2.4
If you take a look at the following article by DMARCIAN, I hope you'll find more details and information you were looking for:
If you want to see what's happening on DNS, just check:
If you set up a CNAME, you'll first see an answer of type 5 (CNAME) and a second answer of type 16 (TXT).