The code starts by using ffmpeg to chop the clip into individual JPEG images. It then turns those into ASCII art printed one frame at a time to your console, using ANSI escape sequences to clear the screen and show the GIFs as you'd expect.
Is this extremely nerdy and limited? You bet. The code is sitting in GitHub if you want to tinker with it, though. And look at it this way: when command lines are almost relentlessly drab, flair like this is bound to help.