You are right about the failsafe, what you're missing is the fact that there are 3 lows and 2 overheads, you guess one tick of hits(kunai+overhead) right, but the other will hit you so fast you won't have time to press or release the down button again.
Sure, it's not entirely impossible, but it's pure luck and you have to tap down so fast hoping that you get it right, since you can't even see what hits you first.