I went to check some of my Touhou expert replays and found some pretty good examples on memorization versus not. One of the replays on Flandre, all the guy does is going in a rectangle box centered around bottom middle of the screen the whole time. He might be moving really fast and it looks like he's doing some totally crazy things, but if you look carefully all he ever does is go around in a rectangular path with varying speed, except the speed isn't random either. You'll see the guy always accelerate to max speed while going verticially, and then slow down halfway while doing the horizontal path, and then accelerate again. Obviously the guy has figured out this is the optimal path to both avoid all the bullets and maximize scoring. You can see his ship always land in the exact same pixel each pass, and assuming no fatigue related issues, you can expect him still going on the same rectangular path an hour later and still dodging everything perfectly.
On the other hand, take Remilia, generally considered as a harder boss than Flandre even though Flandre is the hidden boss. Her 5th pattern, Scarlet Meister, is routinely uncleared on a scoring run. This is because it's both extremely hard, and that clearing Scarlet Meister increases the game's difficulty to a level that will ensure you almost certainly will fail her final pattern, which is worth far more points. If you watch the replays enough, it is pretty clear that you're supposed to start at the bottom middle of the screen, and then go around the right side of the screen in a semi-circle, and then returning to the bottom of the screen at the end of one pass and repeat this until you win.
Except nobody can actually do this because the pattern is fast and spammy enough that it will force you off the optimal circular path every single time. This means you got to improvise each time you go around the circle because your preferred path is going to get cut off at some point and then you got to do some mad dodging skills. Even the best players in the world cannot return to the same pixel as they started each pass. Given enough time, it seems almost certain you will be forced off the circular path completely and then you'll just get trapped in a corner and die. Even top players will make mistakes while going around the circle and you can see the mistakes compound forcing you to do increasingly crazy things the more you get forced off the optimal path to dodge it.
So it's not really surprising that the people who can routinely beat it are the ones with the best reflexes, not the one who has memorized how to go around the pattern. Now you certainly need some degree of memorization, since if you didn't attempt to at least start in the right spot you'd quickly get trapped in a corner and die. But it's not something like Ikaruga or even most of Touhou where if you memorized enough of the patterns, you can be blindfolded and still win. Didn't someone actually do this for Ikaruga? Certainly the guy playing 2 guys at once in Ikaruga relies on memorization.[/i]