I couldn't find any primary source on OpenSSH's licenses, but wikipedia says "BSD, ISC, Public Domain."
Both BSD and ISC explicitly grant permissions to modify the software (and redistribute the modified software), and Public Domain means no rights reserved whatsoever, so the mailing list user's points aren't relevant to any of the Four Freedoms (aka the Sacred Texts).
Without access to the source email: it looks like it's a debate about using copyleft licensing instead of BSD/ISC, which is sometimes considered the Fifth Freedom. If you want an argument about that, I'm happy to do so (later), but it isn't a valid reason for saying some piece of software fails to meet the definition of Free Software.
How does it restrict modifying the software?