The nomenclature was complicated in people's minds by SMTP. Because there was SMTP over a largely transparent encrypted connection, and SMTP where it started unencrypted and negotiated a switch, as well as plain old cleartext. It didn't help that RFC 2487 explained that STARTTLS negotiated "TLS more commonly known as SSL". RFC 8314 explains some of the historical mess that SMTP got into with two types of SMTP (relay and submission) and three types of transport.
And the "S" for "submission" could be confused with the "S"s in both "SSL" and "TLS". It's not just TLAs that are ambiguous, indeed. There was confusion over "SMTPS" and "SSMTP", not helped at all by the people who named programs things like "sSMTP".
I'm still calling it SSL in 2025. (-: And so is Erwin Hoffmann.
* https://www.fehcom.de/ipnet/sslserver.html
* https://manpages.debian.org/unstable/ssmtp/ssmtp.8.en.html