I honestly did not even bother to look at the pre-reqs because that wasn't the point.
The point was that last line. Assuming it is a choice, graze is better.
So, again, going back to your original confusion on why people are so upset with flex and think it is so terrible, we have determined that, if you can use them, every single other mastery is better. That's one major point against flex, sure you can always use it, but it is always the lesser option.
Secondly, and something we haven't addressed, flex removes the versatile property. There is no longer a choice for any weapon that has flex. The entire point of Versatile was the ability to choose to drop the shield to increase the damage. Flex has taken that away. Every flex weapon might as well be re-written without the versatile property. A mastery that eliminates the property of the weapon? That doesn't feel good.
These two reasons are the reasons it is considered a bad mastery. It is, quite literally, "better than nothing, I guess" the Mastery.