Earlier quoted context omitted.
Brain-hand response is not the same thing as a perceived delay. Once latency becomes greater than 50ms, it becomes human noticeable -- aka "not instant." Given that UI response isn't used to trigger human reflexes, you are just misunderstanding the intent.
From my first hand experience in FPS gaming in non-amateur settings the vast majority of average users will not be able to notice a (say) 50 ms difference in UI responsiveness. I covered that when talking about network latency in my previous post. Of course this is just my experience, scientific findings may very well dispute my ignorance.
I see it pretty regularly in playtests and such. Anyway, it's not something someone can simply say "Hey, this app took longer than 50ms to respond!" It's more of a gut feeling that something about it is slow.
They are likely aiming at the 50ms threshold because that's the point at which you really can't notice it, most of the time. At > 50ms, it becomes more and more noticeable, and by just 100ms people actively start complaining about the slowness of response.
edit:
As for the FPS gaming issue, the reality is that you don't notice it in FPS gaming because the games are built to hide it from you -- when you press your fire button, your gun fires, regardless of net latency. And that has been true for a very long time. I think the last game to really wait for server response before doing anything was Quake 3, and probably some of the Quake 3 engine games.
I could never stand Quake 3 for that reason, if you had anything higher than 30-40ms it started feeling like you weren't controlling the game.