Just because less players can abuse him doesn't make him less broken.
If LoL was balanced around bronze (no offense) and worst-case scenarios they'ld have to nerf Garen since he has none.
A realistic (and regularly available) best case scanario leads to much better balancing.
In Zed's case that mean - you can Flash towards your enemy and place your shadow on his head, making your abilities basically undodgeable.
This is regularly doable (often even without Flash) and leads to insane amounts of damage.
Opposed to that we have a best case scenario which is difficult to reach and basically never happens:
Froggen's old Anivia plays come to my mind here. He would manage to place his walls in such a manner that his enemies couldn't but run into his whole combo by abusing the way wayfinding works upon clicking at terrain created by his wall.
If have seen dozen of diamond players who continiously failed to pull it off.
This is a best case scenario which happens simply far to rarely to be relevant since it requires thousands of games of practise to be pulled of on regular bases.
- Opposed to above Zed played which can be done regularly if your opponent simply steps a little bit to far towards you.