It would be a rather poor mechanic if, as soon as you hear the pwerfer fire, you retreat, but you still lose a squad or two because they took to long to move.
But thats the point.....A big blob should be hindered and suffer a slower reaction compared to small groups of units.
If you read my post it suggests that the retreat time is only delayed if you blob (whatever we could describe that as) which means that anti blob weapons can do further damage.
if you are just using a couple of squads for example then the reaction time would be the same.