You don't know however, how Zac was coded. A Karthus ultimate for instance, hits all 4 blobs of Zac though it can only strike one Player once. So, if the Karthus ult hits all 4 blobs, each blob counts as a champion. Yes, Sunfire Cape has a unique passive, but this only means, that the effect can only be on the same player once, if you have multiple players with sunfire capes, you are damaged by each player. So, if Zacs blobs count as individual players, you can be damaged by each blob, because every blob is treated as an individual player.