There was a cable-based serial terminal networking system called LocalNet, which was all over the University of Illinois Urbana-Champaign campus circa 1985. The LocalNet modems/concentrators connected terminals and computers throughout the campus. You'd fire up your terminal, hit ENTER for a prompt, then type CALL xxx,yyy (xxx and yyy being hex addresses) to reach a particular host. The host just saw it as just another RS-232 modem.
I'm not an expert, but I'm pretty sure you're correct. Interestingly though, I have an PC I got from a college that has Win 3.11 and has a network card in it and TCP/IP isn't enabled or on it. I was trying to figure out how to use the network card, but all I can find are a bunch of other communication standards.
I think you needed Trumpet WinSOCK to get 311 on the internet. And there wasn't much other than email, and Netscape to do with it back then. Gopher maybe, but I didn't hear about what that was until it was more or less dead.