It's only checking that the original tstate *for the current thread* and
the new tstate have a different subinterpreter.  A subinterpreter can
have multiple tstates, so long as they're all in different threads.

The documentation is referring specifically to the PyGILState_Ensure and
PyGILState_Release functions.  Calling these says "I want a tstate, and
I don't know if I had one already".  The problem is that, with
subinterpreters, you may not get a tstate with the subinterpreter you
want.  subinterpreter references saved in globals may lead to obscure
crashes or other errors - some of these have been fixed over the years,
but I doubt they all have.