That doesn't really explain it. Also, Iaijutsu doesn't require the enemy to be unaware or anything. Shinobi's gripe is thus:
Swinging your already-drawn sword to attack a foe: Complex Action
Using Iaijutsu to draw your sword and then do the above: Simple Action
If it was "You may draw your weapon in the same action as you attack with it as a Complex Action", then it would make sense, because it's just shortening the time to draw a blade and that makes sense. What doesn't make sense is that the moment after you've made your Iaijutsu swing, your next swing suddenly takes about twice as long, for no reason, in addition to the first strike even including an extra action. It's not unreasonable to ask "If I can unsheathe and swing my sword as a simple action, why can I not just swing it as a simple action?"
I can say why it's written that way; it's based off the already weird Quick Draw action on page 165, which lets you draw a weapon and attack with it as a simple action, including melee weapons, but they have to be small, so usually it only works with knives. Iaijutsu is just the technique to do it with a weapon of any size as long as it has a sheathe of some kind.
It's saying "you take a simple action to draw it, and if you succeed on this test, you can still attack that turn". The rules are just dumb; instead of shortening the action it takes to attack with something called QUICK DRAW they should have made it a shorter action to draw the weapon. 'Cause they didn't weird stuff like this happens.