From a storytelling point of view, Oda DID include extermination of Kurozumi clan as the motivation for current events. He also was quite explicit is showing that people of Wano committed atrocities too.
In the end, only atrocities committed by one side were resolved and other never touched upon. How is that good storytelling?
....Why are you putting words in my mouth lol? When did I say it's good story telling?
I know this, I'm speaking objectively about said tasteless pun. The point is, context is important:
There is a story being told about how Hiyori faced Orochi when he tried to burn her with him.
It then ends with her "literally" stating that coals are meant to burn as a pun. She is saying this to Orochi after he is set on fire. It sounds like it's being said in a flashback and everyone is repeating the line in the present.
Again, it's a poor joke, and for the 3rd time, I agree. But you are missing pictures and context. I guarantee you delivery of this line won't be so heavy handed to imply Wano wants to commit mass atrocities to a cursed family.
I also guarantee you mangaplus will probably just say "Coals are meant to burn" without context to the pun. Watch.