It would be bonkers not to use your wheel speed as your speed but seeing as Bkool uses your wheel speed to calculate power coupled with the resistance set there could be some mad logic in using your power meter reading and the 'conditions' (weight, slope, road surface) to back calculate speed but that's like admitting that your algorithm to calculate power was wrong in the first place as your power, even with a power meter, is proportional to the resistance set.
Wouldn't we just love to have an open forum with the script writers!