I recently moved from Kitty+Tmux to Tmux only setup. I think the maintainer has implemented most of the features in tmux like split panes, switching, changing layouts etc and is probably why says what he says.
I've been a long tmux user, but I've migrated to Kitty only setup. To make my muscle memory feel at home, I configured Kitty with nearly the same keybindings as tmux. Here is my setup https://blog.funcer.xyz/blog/kitty-terminal/
I appreciate his efforts, but tmux from my cold, dead hands. kitty is nice and fast, but tmux is in my blood (and on remote servers where kitty isn't).
For me, I've found Kitty quite configurable enough to have everything except the remote server thing. I used this as my guide: https://www.reddit.com/r/KittyTerminal/comments/z2p2sh/ditch...