For fitness there are more healthy athletes there keeping there body in shape and less people who are overweight. Spinning session you got about 10 or more bikes with professional cyclist making mayor amount of power.
Plus I dont think you can use BHP to calculate the amount of energy that is produced. Cause the spinning or training cycles got gears to change the difficulty of the training and also change the speed that the wheel/dynamo spins.
If there are gear that change the movement for higher speed that makes more cycles are turned in the dynamo and so more energy is produced. And a self-excitation dynamo can produce more energy then a normal bicycle dynamo cause it use a magnetic coil that can produce higher magnetism then a normal piece of magnet.
So there more factors that need to keep in mind to produce energy.
Simple bhp to watts got nothing to do with this. You can’t calculate these things that simple.