[JdeBP]

No, they do not. They put a domain name whose human-readable form just happens to resemble the human-readable form of an IP address into the record. An actual machine-readable IP address, and the actual machine-readable domain names that go into MX resource records, are quite different things, and one simply cannot put one into the other. This has been pointed out several times in this discussion.

It has been further pointed out that even if dealing with human-readable forms in (say) BIND's "zone" format files, the human-readable form of an IP address doesn't actually get translated into a 4-label domain name, because it isn't a fully-qualified human-readble domain name. Human-readable IP addresses don't end in dots, and if one has typed in a human-readable IP address into a "zone" format file, one actually has not made the relevant misconfiguration; and conversely if one has made the misconfiguration (in a way that will actually work with dnscache et al.) what one has typed is not actually a vanilla human-readable IP address.

Ironically, neither the headlined article nor many in this discussion have noted that the SMTP standard's algorithm for loop prevention breaks when one does this in MX resource records for the same reason that it breaks when one uses alias domain names via CNAME records rather than the canonical domain names. SMTP Relay servers do not match the domain names in the data of MX resource records against (their own) IP addresses because ... well ... those domain names are not IP addresses.