@p >What makes you think that?
Since anyone who isn't a loser uses the best shell.

>Linux supports the necessary Unix interfaces to be called a Unix
As noted below, it does not.

Unix™ is a trademark that requires payment and certification.

>Would you prefer I use a different compiler and link against a different libc?
That would be quite sad, but that wouldn't be Linux either.

>Doesn't look like anything by that name shipped with v7.
Yes, because clearly the free software replacement, GNU cal, came later.

>I wouldn't call it proprietary: the license required it to be distributed gratis and with source.
If you cannot sell the software, the license is a nonfree license, as freedoms 2 and 3 are violated; https://www.gnu.org/philosophy/free-sw.en.html#four-freedoms

>This code contains a firmware binary to overwrite the binary firmware that was already in the machine.
It is NOT firmware, it is SOFTWARE.

Whether it works as a software patch against instructions in ROM is irrelevant, as it is software and it is proprietary.

>Whether or not this constitutes "linking" is murky, as I understand it.
That is irrelevant, as the proprietary software is still in Linux, making Linux proprietary software.

>Of course, Plan 9 is GPL'd.
Not anymore.

An old version was GPLv2-only for some parts, but later versions are under different licenses.

>If anyone gives you a copy of the software in binary form and does not supply the source, you can get the source from them, by the terms of the license.
The problem is that the Linux developers actually encourage proprietary software developers to write derivative works of Linux and distribute them in binary form only and refuse to provide the source, by never enforcing their license against many of such cases.

Although, when it comes to free software in source form, under a free software license that better enhances the users freedom, under the GPLv3-or-later, suddenly those Linux developers start to enforce their license?