I see he generically refers to "the mod key" but I don't see anything declaring what the mod key should be like you would in a tiling wm config, so I'm assuming the M- is the meta key in all these keybinds then. I wish that had been made more clear. I was pretty curious the whole time I read the article how he'd changed C-b to something else and what he'd picked. I had also briefly wondered if he'd somehow made things modal like vim so he could enter some sort of "tmux mode" where he could just press a key for an action and then return to his insert mode after. (also a bit like i3/sway's modes, the most common one being for resizing windows)