Now of course there is a sense in which they were – some mainframe installations supported thousands of users! But there is a distinction.
Consider the familiar fixture in so many 80s computer science labs: a VAX running UNIX or VMS, with an account for each student. That's a multiuser system in the sense that each user had an account with the right to run arbitrary code. Thus it was necessary or at least highly beneficial for the operating system to use memory protection to screen the users from each other, so that e.g. one student trying to debug a C program dereferencing stray pointers, couldn't crash another's processes.
By contrast, consider this site, retrocomputing.stackexchange.com. It has many users – but we are not users on the underlying server. The server is running an operating system, most often Linux, that has multiuser capability like the VAX – but that multiuser capability is not being used. We users do not have the right to run arbitrary code, and we can be handled purely in software. To the operating system, the entire site is just one user.
Now, IBM mainframes were not really about providing student accounts. Their stock in trade was transaction processing. A mainframe might, say, run an airline reservation system. Each teller making bookings would be a user within the reservation system – but not running arbitrary code, and not a user on the operating system.
So that's the typical mode of use of an IBM mainframe. And at least in the early days, some of them were also used to run student jobs in batch mode.
But were any of them used like the college-lab VAX, to provide a bunch of people with interactive shell accounts that could run arbitrary code?