Yes, the FAA has ruled that is the case. That's why travel must be "incidental" You can't advertise even free flights with out an air carrier certificate, unless you were already going to make that flight for your own reason besides flight time.

The FAA has ruled that a CFI instructing is being compensated based on their CFI not their commercial certificate, which is why a CFI need only a 3rd class medical ( or none if the other pilot is current and qualified for the flight and acts as PIC ) But that CFI is indeed still being compensated as a pilot.