A carnivore diet is not necessarily a ketogenic diet, although it is a zero-carb diet. If you eat enough protein then your body will make glucose from it and keep you out of ketosis. But I agree that a diet of only red meat is absolutely ideal no matter which angle you look at it from.
I didn't believe it either... it seems to be mentioned in a bunch of keto literature online, like this one [1]. Some folks say it stimulates insulin, and some say it upregulates gluconeogenesis. I haven't found anything authoritative, so I remain skeptical.
So, it's just a thing you think might happen, because there is such a thing as gluconeogenesis? I went looking, and the only cite I found was a study suggesting this does not in fact happen.
Basically a keto diet, of course it works due to ketosis.