I was low platinum last season. Not anything special in my opinion. But I faced Lux's who knew how to play the lane match up. Basically I could only kill the Lux's that didn't know how to land their binds against me after I ulted. Zed's ult, which is his only method of actually securing kills, lands you in a pretty predictable spot. Right behind you. So as he lands, you can just Q behind you and he's basically unable to even touch you. So basically he can't dive you in teamfights or under tower in lane, and thus the only way he can kill you is if you get caught out, which is your own fault. In addition to this, I was never able to land a full poke combo because of their range and zoning potential (Lux's E is impossible to dodge if you're in melee range). I was only able to land Qs that were also thrown from my shadow.
And even if by God's luck that a good Lux player misses her Q on you after you ulted, generally she will take exhaust or barrier to basically negate your burst, making diving her still impossible even if you dodged the key CC she has.
Zed can beat a lot of mages like Syndra and maybe Orianna. But not Lux, or at least not if the Lux is on an equal skill footing as Zed.