The missing part in your description, to the best of my understanding, is fat to ketone conversion - you don’t use the fat directly, the process takes time, so ketones have to already be circulating when you need them.

It can also explain why ketogenic diets work for many people - if you exercise every few days, your body will maintain sufficient blood ketone level for your exercise days, but unlike glucose, ketones that are not used get peed out eventually (and not stored/converted back to fat).

Couldn’t find anyone knowledgeable enough to confirm I am right, but also no one to tell me I’m obviously wrong - still looking for one of those answers.