That's way too simplistic a way to think about it. There's getting to an actionable state, physically inputting the punish (input time), the move's startup, and the time it takes to get close enough into consideration. Also you can't necessarily tell someone is rolling from the first frame, so the first frame that confirms that a roll is occurring matters as well. All these things add up.
And of course you should know that that figure is based on non-choice-based reaction, right? You'd have to be standing still and completely focused on watching for the roll spark for your model to be even remotely tied to reality.
So it comes as no surprise to me that in Soul Calbur 5, nobody can block a perfect Yoshimitsu iFC3K (i22 if done perfectly) on reaction (yes this includes their top players), even though the first few frames are a tell (you have to crouch first and then press down-forward kick). And blocking low only takes one frame. There are tons of other lows in the game that are between i15 and i20 and are "unreactable" as well, and it's a running joke in the community to claim you can block them, since it's a classic noob mistake to overestimate one's reaction time.
This "Average human time is 15 frames" figure would have you believe that you can block standard i15 low kicks on reaction. Hah!
Of course, there's still enough time to punish rolls that end up very close to you. What, you dash grabbed someone who rolled, and you started dashing after they started rolling? They probably were just shielding, you didn't get the punish, you just guessed right on the mixup. To really beat rolls that aren't landing right next to you, you need to be consciously covering that option,among others, if your character can, and be in position to punish.
There's more to it than "lol my score on
http://www.humanbenchmark.com/tests/reactiontime is below .500s/30f."
That has nothing to do with reaction time.