Engine on initially with EV mode, never get 199mpg

In my Mazda 5, I often get 88MPG or so during engine braking and 99.9 during coasting.

In my 2004 Prius, engine braking also didn't result in infinite mileage, as determined by how fast the average increased.

In my Prime, engine braking triggered by regeneration causing an ICE start most certainly uses fuel, as it runs through the warmup cycle during the engine braking.