From what I've read, they state that "some dogs" have toxic reactions to raisins or grapes, causing their kidneys to shut down but they've yet to identify what~exactly~is in the grapes that would cause such a thing. Since it doesn't happen to all dogs, I'm thinking some dogs are genetically prone for kidney failure and just happen to have eaten grapes~not something commonly on a dog's menu~and they pick out that one component and blame it for the renal failure.
I've lived on this Earth a long time and have never met anyone in real life that has ever had a dog die from grape poisoning, so it can't be all that common. Maybe just one of those things that get repeated over and over but has very little basis in fact? Sort of like how if chickens eat apples it affects their laying...but no one stops to think that apples fall to the ground in the fall, right when laying is slowing down anyway. Purely coincidence.
Or that apple seeds have arsenic and can kill your chickens if they eat them. GREEN apple's seeds have higher levels of arsenic but rarely are chickens going to be eating unripened apples and still wouldn't ingest enough green apple seeds to get a lethal dose of arsenic. If they did, can anyone say without a doubt that the chicken died entirely due to the apple seed consumption and not because they already had underlying health issues that contributed to their death?
I do know foxes eat wild grapes all the time, so unless they have something dogs do not have in the way of neutralizing the toxic effects of grapes, I'm just not convinced they are lethal to canines.