It's not, it does work for video as well. I have done it myself, and there is no secret, you just run mpv on a video, in the TTY, and it works. Though it may not work inside of fbterm, and it will definitely not run inside of tmux unless you do some trick to detach and run it in an external shell and then reattach.

Haven't tried that myself, because again, I'm probably not abandoning X, because of the amount of control that I have over everything. Like, I wanted mpv to automatically fullscreen itself, but not in every workspace. I wanted conditional keybindings that do different things depending on the focused program. I wanted a prefix key to open other key maps than can then open other key maps themselves, so I can technically bind a billion things to one key. All done.

Things like that are definitely more doable in X than outside of it, and they are definitely easier with StumpWM than with almost anything else in existence (and it already basically behaves like a terminal multiplexer anyway, which I like). Then there are also tools like xdotool, xcape, xmodmap. And all of that can communicate with terminal programs as well, using tmux as a bridge. I can set bindings to send other bindings to tmux windows that don't even have to be attached, kinda turning programs into daemons, in a way.

Basically, with my current power level, I may need EVERYTHING duct-taped together, to have something that even approaches being acceptable.