Ahhhh, so I was trying to use Debian terminology on an Ubuntu install, that was my problem.
(Also why using 'sudo apt-add-repository' from the command line is superior to editing sources.list -- it will TELL you when you fuck up, instead of just letting you save the mess and regret it later.)
Anyway, I have learned something new, and also the directions I was trying to follow still don't make Steam work, so. 🤷
This is like the time my wifi didn't work for six months because of an update.
Aha, I just had to go to the hidden folder .steam and run the installer shell script there from the terminal, and then it finished doing the install (and deleted the shell script). Now the client will run.
This seems like the sort of thing you'd add into your install process instead of requiring users to do separately, without instruction, but what do I know.