Nessus and SMTP clear text logins false positive

I just ran Nessus against a client and got this as a low finding:

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Synopsis
The remote mail server allows cleartext logins.

Description
The remote host is running an SMTP server that advertises that it allows cleartext logins over unencrypted connections. An attacker may be able to uncover user names and passwords by sniffing traffic to the server if a less secure authentication mechanism (i.e. LOGIN or PLAIN) is used.

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This was on port 465 which is SMTPS. To manually check the finding, I initially tried to connect using netcat but the connection didn't work correctly. Then I tried using openssl to connect and that worked and gave me an encrypted connection. The EHLO command then returned:

250 - AUTH PLAIN LOGIN

I understand that that config means the creds go across without any obfuscation, but as the whole connection is wrapped in an SSL tunnel, they aren't really sent in clear text.

Am I missing something here, or does this seem like a false positive? What I think may have happened is Nessus has connected to an SMTP server using a library call which hasn't passed back to the plugin that the connection is encrypted, the plugin sees the login option and assumes clear text.

I would raise this with Tenable but whenever I raise something like this they want a copy of the scan which I can't give them due to NDAs.