I wonder why are these called "threads" then.

The very point of threads (as opposed to parallel processes) is the easy access to common memory and other resources.

In fairness, that is in the pipe for python for a release soon. One interpreter per thread.

Not quite, subinterpreters can contain multiple threads each. But you're right in that if you limit yourself to one thread per subinterpreter, then you should be able to get good multicore scaling that way, when more fully fleshed out subinterpreters arrive in 3.12. The trick then becomes how to efficiently communicate between them. Interesting times ahead.

Erlang is fairly famous for working in a similar way.