It’s all a matter of perspective.
Maybe even smelling something sweet (a person near and dear claims that she can actually tell salt from sugar apart by smell) could kick one cell per few thousand back to sugar-burning – would we then consider the fast to have been abruptly terminated? Because I bet that there will always be such “stubborn” cells, even when we’re in “triple-plus ketosis” according to our app, heck, even when we’re nearing life-threatening ketoacidosis – there’s no state of zero glucose utilization capable of sustaining life (same holds for ketone / fat utilization too – there’s always some baseline degree of it, even when we’ve just had that sinful donut).
No, seriously, enjoy your coffee.