It's not an alternative to tmux, it's an alternative to the macOS Terminal.app or iTerm2.
You can run tmux inside Ghostty.
To be fair, you can also run tmux inside tmux (inside Zellij, inside another tmux, inside screen, etc). :)
For those who will read this literally, it works fine on linux as well.
To be fair, you can also run tmux inside tmux (inside Zellij, inside another tmux, inside screen, etc). :)