As far as I can tell, WFP never said they could "solve" world hunger with $6B. They did put together a report about what they could do with $6B, which didn't seem to interest Musk after all.
CNN maybe wrote a bad headline? Doesn't really seem like the sort of thing to pretend to offer and then deny 42M people food aid over, but then I'm not a trillionaire. I don't think he bothered to actually read the story, instead just tweeted after reading a screenshot of a headline. He got mad about something the WFP didn't even say, and then people decided they must have said that because he got mad about it. They didn't!
If he really wanted to know what $6B could do, he could have donated it and found out. It was cheap enough for him to donate $6B to sit in the "foundation" that he personally controls, so it's not like he was hard up for cash.
https://www.cnn.com/2021/10/26/economy/musk-world-hunger-wfp...
> Correction: An earlier version of this story’s headline incorrectly stated that the director of the UN’s food scarcity organization believes 2% of Elon Musk’s wealth could solve world hunger. He believes it could help solve world hunger.