Going to have to ask for your source on this. PLD emnity hasn't changed in FOREVER, and it's been theory crafted before that Flash is far and away the weakest emnity generation (multiplier was mathed at 2x) in comparison to RoH (mathed at 5x of damage [x2 if in shield oath]).
http://valk.dancing-mad.com/methodology/#Enmity.
I realize this link is quite old, but I haven't seen anything worked up recently, likely due to enmity not changing within the game. So if there is a new source for this information I would love for it to be known.
From my understanding, Flash only generates more enmity if you are doing a non combo'd RoH, as the majority of the enmity generation using RoH is based upon the damage it is doing.