Besides, Pymux is Python while tmux is C. I'm sure you can appreciate that adding a Python API to tmux would be non-trivial, both code-wise and politically.
You can see the quickstart at https://tmuxp.readthedocs.org/en/latest/quickstart_python.ht... and the API documentation at https://tmuxp.readthedocs.org/en/latest/api.html.
You can see the testcases we cover use real tmux sessions and monitor the state of the session / window / panes to assert the API functions correctly. https://github.com/tony/tmuxp/tree/master/tmuxp/testsuite
As for pymux, Jonathan Slenders' work on ptpython [1] and prompt_toolkit have been quite nice when working with shell_plus in django or debugging python projects. He's good in this area. Agreed that it's a nice thing to have pymux there.
And agreed that adding native language extensions (python, ruby, etc.) to tmux would be complicated code-wise and perhaps not in the spirit of what it aspires to be. That said, on the tmuxp side of things, a lot could be improved. For example, switching tmuxp to use control mode (-C) and filing smaller patches to expand control mode's events in tmux(1).
tmux is comparatively rock solid.
I personally find this project interesting to see, although I won't be using it.