Great point, plus the body can make glucose on its own if it's needed, so those keto people still have glucose, it's just 99.9% of the body is burning fat, so there isn't much demand for that glucose.
Bonus fact: the emerging literature is linking long term neurodegenerative diseases (dementia, Alzheimer's, parkinson's, etc) to impaired brain glucose metabolism... I.e the more insulin resistant someone becomes the more energy starved their brains are, but even in those people ketones feed the brain without impairment