I didn't follow every detail of the discussion but I suppose one source of confusion is the difference between power expenditure and power to the pedals. For humans, power at the pedals is no better than ~25% of the power consumed, with the balance being heat.
So while 300 watts of calorie burn rate is very doable for a long period, delivering 300W to the pedals is not possible for all but the most elite.
So while 300 watts of calorie burn rate is very doable for a long period, delivering 300W to the pedals is not possible for all but the most elite.