He is lucky that he did not suffer more permanent and debilitating injuries, while bicycle riding as injuries go; in the DFW area alone there have been many cyclists’ troubles, including pedestrian/jogger into bicyclist and also into each other.
There are many other modern gyms with great alternatives and better controlled environments, so it’s somewhat careless to expose ourselves to danger if you are schedule for such a privilege opportunity.
Dave is certainly correct, of course, the difference depends a lot on how hard one rides the bike. Pedaling exercise is good for a few muscles, but the serious effort of keeping the bike under the rider uses a whole lot more muscle groups. At least, for me it does.
But training on a stationary bike does not provide quite the same level of training and physical tuning as a normal bike - an important distinction for athletes such as the astronauts. This is somewhat similar to the difference between running on a treadmill and running on a track or street.