That's a unique instance, since Cinderhulk was a strong item both in the jungle and in top lane. Riot is known for disliking jungle items in the lanes. If one or two champions are able to do it they are fine with that, but when smite pushes other summoner spells from a lane, purely due to an item, there's an issue.
In that specific instance the item should have been nerfed before the champion because they wanted to stop the item from being so efficient in lane. Had the item not made its way into top lane, then I do think just nerfing sejuani probably would have been enough.
I do agree though that nerfing sejuani and cinderhulk at the same time was extreme though, as it seriously hurt sejuani as a champion. That was a mistake on Riot's part.