My brain was wondering how a video of only 0.2 seconds would work. Can you even say an entire word in that time? And how is that enough to explain all of Super Mario Bros. 3 TAS?
I was kind of disappointed to see the video lenght makes sense, and my brain was just not awake yet.
Don't know if you ended up watching it, but effectively they're able to randomly execute code during the loading screen using a bug caused by audio generation vs controller polling. So by using TAS tools they can spam the controller to cause an audio bug that let's them jump to executing code from Ram that they are able to manipulate using controller inputs.
TLDR: Using a bug in the audio processing, they can use controller inputs to write arbitrary code that jumps them to the end of the game.
TAS is cool as people can use it to find the most optimal way to finish a game. Sometimes tricks are too hard for people to do or at least they are believed to be to hard for people to do. Sometimes TAS “only” tricks end up being done by people.