I don’t know for sure, (so nothing to back this up but bro science), but I guess it has something to do with training your heart and lungs to provide enough oxygen for your total body (high vo2max). In addition, the muscle fibers in your legs are already primed to do the kind of work you do in cycling (endless fast reps)
If you then move to cycling, you will have more oxygen available then the average cyclist, the muscle fibers have the right composition, so you mostly have to train an efficient pedal stroke, but that might come easy to as rowing also requires timing and stuff like that.