If rhythm games were all memory, I'd be even worse at them, I can do well enough with my extremely dull senses in Project Diva, but I ain't breaking any records or Perfecting more than a few songs on Extreme once in a while if I get lucky.
I used to play Stepmania and Pump It Up, there were songs I could play with invisible steps, so I kinda know that if you're really into those games, it wouldn't take too much work to do this.
In reality the player probably memorized it all, if you deactivate the effects, you can play the song normally and memorize them. After that, it doesn't really matter how much the screen shakes or turns.
If you pay attention nowhere during the song the amount of steps was too high or too fast, so its not that hard to memorize (for those who are into it)
AGDQ always has unbelievably skilled players, the best really. Even the worst speedruns make me feel like a complete newbie in games I've been playing for years now.