I absolutely abhor this quote, I think it's taken out of context.
First of all, it denotes hubris, which I hate.
Secondly, it's totally anti-scientific, it assumes that Unix is some paragon of OS design, the zenith of Computer Science.
I like Unix as much as the next IT guy, but let's cut it out with the cliché quotes. There's plenty of things Unix doesn't do that well (everything is a file, until it isn't; filenames are basically binary blobs, etc.: https://dwheeler.com/essays/fixing-unix-linux-filenames.html).
Plus your comment is just silly, whoever implemented this obviously knew about Unix shells... It takes knowledge of Unix shells to implement another Unix-y shell.
And "non-portable" because it's only portable to Unix? That's... Approximately everything. Linux family and Darwin naively, NT via WSL/cygwin/etc., BSDs obviously, Android sorta-natively or via Termux, Chrome OS via Termux, crouton, or crostani(sp?). What is Unix not portable to?
Your actual point, of course, is valid regardless:)
EDIT: Oops, actually https://en.wikipedia.org/wiki/Small_Device_C_Compiler ... but then you're right about getting pretty obscure.
Yay! Non-portable Unix tools! Just what everyone wants.