It is in a way, in others not though. As soon as I'm on a bike I have to go fast. It's like when you hop on a train and you just want to go from A to B quickly. Speed matters. My average pace on a bike would be around 20-25km/h (bout 12-15m/h?). That is by no means super fast. Heart rate will go up of course and I will definitely break a sweat. Still wouldn't say it's exercise. To me exercise is putting time away from my daily routine to specifically train my body. But that's of course a totally subjective take on this. I agree with you in the end though. Guess you could say it's exercise from a certain point of stress on your system.