The vehicles may have been able to use E85 because not all vehicles that can use E85 are labeled “flex-fuel.” Some flex-fuel vehicles only have a small emblem on the side or rear of the vehicle, while others only state that the vehicles can use E85 by looking at the underhood emission control sticker.
I have had students do the same because E85 was cheaper and had a higher octane rating. It will, however, make the engine almost impossible to start when cold. It will also run lean, making the engine stumble and it may stall. I told my students to just drive and top off the tank with regular gas every 50 miles or more to try to get the tank back to gasoline. I doubt it would hurt the engine, but it will not run right and the fuel economy will be about 20 percent lower. For example, a Chevrolet pickup truck that is EPA rated at 15 MPG in the city and 20 MPG on the highway using gasoline is rated at 11 MPG in the city and 15 MPG on the highway using E85.