Things that are effective in a pro player environment with coordinated 5 man squads are not necessarily (and usually aren't) effective in solo queue, and what champions are capable of in the hands of some of the most mechanically gifted players in the world isn't necessarily indicative of the strength of the champion in non-pro play (even including relatively high elo play).
As a result of that, the question isn't "Is this champion balanced?", but "is this champion balanced at a particular MMR in a (solo/team) environment, and how oppressive/useless is it outside that environment?" since being balanced at all MMR's for both solo and team play is not realistic for any non-trivial champion.
In some cases champions might be fine at most levels of play, but get completely oppressive in the hands of gods (e.g. azir) to the point where they need to nerf it, but that can leave the champion basically trash for everyone who isn't a god. The gods can probably still use the nerfed champion an be on a roughly level playing ground with other picks, but 'other picks' likely have a lower skill ceiling so they generally won't bother as theres a bigger margin for error.
Many of the champions from this worlds received a number of nerfs leading up to worlds, and weren't necessarily oppressively strong in worlds. There were some post worlds nerfs/bugfixes (protobelt had a bug which was fixed after worlds for example) which may have also impacted those champions strengths without directly nerfing them. All the preseason stuff also has had lots of rippling effects, so things strong in the worlds meta may not be as strong when theres a fresh batch of assassins around and the jungle is a bit different and various other things.
Basically, they might have seen their power levels in worlds and saw they're stronger than ideal, but deemed them within acceptable boundaries since the game isn't balanced around worlds level play. Or, they may have seen their power level at worlds, and felt they were out of line, but wanted to see the impact of preseason changes first before making direct nerfs.
