I thought AppleTalk and file sharing was standard on pretty much all Macintosh models going back to the early models. I guess not...
Once you've enabled AppleTalk on both computers and connected them with a cable, they're networked
Yup, this. Or, to put it another way: there's a difference between being able to send messages from computer to computer and being able to share files between them, and the first has to happen before the second can. And there is also a difference between being able to access shared content from a computer and be able to actually share that content without any extra software. Perhaps as an analogy here, pretty much every modern OS comes with a web browser, but not all of them come with a web server. "I can use shared volumes and file shares from very early machines" and "I can't share files until System 7" are not actually contradictory statements.
Perhaps tiresome detail:
This was especially true of the older Macs, where resources were sufficiently limited a lot of the time that you would almost always want to put aside a whole computer to do your file sharing server duties, especially given the lack of multitasking on the early Macs. And as
@Dog Cow mentioned, the
original plan for this was to have a whole separate product for this use, which would just sit there and JFDI, kind of like a networked external hard disc. In fact, if I remember correctly, for a while this was
the approach Apple was going to take to hard disc storage.
After that plan, there was the early AppleShare server software, but this required a machine to be dedicated to it. Again, think of the analogy with the web server here; the software was
available quite early, but not bundled. It wasn't until System 7 that being a file sharing
server was bundled with the core OS. And it isn't a coincidence that this came along at the same time that multitasking became non-negotiable. File Sharing under System 7 is actually implemented as a background-only
application that runs all the time. And this in its turn is because Apple were only targeting Macs that had enough grunt (especially memory) to be able to actually multitask, which also by knock-on means they had enough memory and grunt to mostly share files.