The 200ms braind-hand response time you are refering to is the time it takes to see a change on the screen, brain reacting and sending a signal to the hand and then to actually press the button. What the firefox roadmap is talking about is from the time the button was pressed until something changes on the screen. These are two different things and they actually add up. Then there's also the final delay between the changes happening on the screen and the brain recognizing this.

So the user experience delay is the sum of three different sub-delays. Firefox wants to improve it by reducing the middle part.