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).