> Arguably CMake is a build system; as a user you can build your project only by running CMake commands.

If you put together a shell script that calls some other program that runs a build, that doesn't make shell script a build system.

> The fact that it will run e.g. ninja or make under-the-hood can be ignored for many use-cases.

You can't, though. Some features are only available on projects generaded by cmake for specific build systems.