A sprint is running as fast as you can for a short distance. Obviously that will look different at the end of a 3hour jog than it does in a a 90min football match.
Also there are more endurance sports in the world than marathons. Many of them have formats that require short bursts of intense output that feel very similar to sprinting.
It seems like the distinctions are not totally clear cut, but the point is that the muscles and exertion of jogging burn much more fat and ketones in a keto-adapted runner. The muscles and exertion involved in sprinting and, to a lesser extent, running, require glucose as fuel.
The important differences aren't how fast you can run, how it "looks" or how it "feels", the difference is which fuel the muscles are using. On a high-carb diet, if you're too exhausted at the end of a 3 hour jog to do a decent sprint, then you're probably not changing the fuel you're burning for what feels like a sprint to you.
But, if you're on a LCHF diet, for 25 7/8 miles you'll be burning mostly fat and ketones all along and have some glucose/glucagon in reserve, for that sprint to the finish. (Kind of like the caveman who's been running the mammoth to exhaustion with his hunting party, but has enough glucose in reserve for that life-saving burst of speed when the prey turns on the hunter.)