But wait, I’m a user of tmux or zellij which for me was basically another tmux and I went back to tmux eventually. I’m not looking to develop another tmux or something. Is this project more of a library to create tmux like apps?

As of today you can replace tmux by rmux. Its behavior should be 1-1 iso; tmux is more stable though, it has been battle tested with a dozen of years of edge cases refinement. rmux is a faster and more elegant clone of tmux + windows support + programmable sdk