That's a totally different thing. Native macOS app vs portable terminal multiplexer. My main use case for tmux is detaching and re-attaching to a session on a remote server, for which it's extremely useful.
That's what I tell people who keep telling me to try cmux. It's false advertising to say it's like tmux. No, Zellij, sure. But not this. I will hold onto tmux forever and you'll have to pry it from my cold dead hands.
what does it add over screen which i don't even need to install?
I've been building a tmux wrapper that is similar you might be interested in. https://jmux.build