Your definition of overdrive is slightly incorrect. Overdrive simply means that the overall ratio between the input to output shafts of a transmission is less than 1.0 (meaning the wheels of the car are spinning faster than the engine).
By this definition the Miata does have an overdrive as most generations have a final ratio of around 0.85 in top gear.
Your definition is correct, but your conclusion is incorrect.
The wheels of the car are not spinning faster than the engine (in the overwhelming majority of cars) because there is still a final drive ratio between the output shaft of the transmission and the wheels. That is rarely taller than 2.50:1 and often greater than 3:1.
Look at the revs per mile in a tire's specification, drive that car at 60mph, and see that the engine RPM is higher than the revs per mile figure, even in the tallest overdrive gear that you have.
By this definition the Miata does have an overdrive as most generations have a final ratio of around 0.85 in top gear.