He didn't design Zed or Fiora. He also didn't design Azir, who is either broken or too weak. He didn't design Gnar, the gatekeeper of toplane or Camille.
Riot design philosophy is just to create unique and fun to play champs then balancing them with numbers later. I like certainlyT's sense of fun. I know a lot of people who hate support, but when they get autofilled the lane they have a champ to fall back on. It's Tresh btw. A certainlyT champ. I don't care if you hate Yasuo. He is fun. I don't care if you hate Zoe, she is fun. I like Warwick, he is fun. I'd argue those 3 champs aren't even broken on a design level as they do have weaknesses, but it is undeniable that CertainlyT champs are unique and most importantly fun.