That looks wonderful! Being able to place the config files in project folders is a great idea. It always annoyed me when I forgot to backup the magic config folder in tmuxinator when moving machines.
Also tmuxp allows you to save the settings from a running session into a config file. That's gold.
Any comments on how it differs from tmuxinator ?
Mostly, see my response to mnem.
Also, tmuxp is python instead of ruby, and it gives you an API to interact from your python code with tmux.