It is widely considered a "sub-domain" but I'd argue it is not, but a "regular" hostname (in case of aforementioned record types) instead.

Reason for that logic would be that a (sub-)domain is a domain/realm of entities (records managed via its nameservers). An A record would not satisfy such a definition.

Taken from Wikipedia

> A domain name is an identification string that defines a realm of administrative autonomy, authority or control within the Internet.

Though Wikipedia also follows the standard definition - https://en.wikipedia.org/wiki/Subdomain

I wonder if this is a similar misnaming as when people regularly confuse traffic and bandwidth.