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The main argument would be that the pro is meant to already be at their ideal weight or within a few kg of it, and certainly putting out more power if they are on the heavier side of it. See Thomas De Gent as an example: challenged for the Giro, put on weight one winter, tried to lose it and maintain power, couldn't.
Comparing middling club racer to a pro who is meant to be riding for the best team with all its marginal gains, is like comparing apples to bloody beach balls
Its nonsense
Years ago, I went from 73kg to 68kg (I'm 6 foot / 183cm) over three weeks cycling in the Himalayas, while getting stronger. Came back and won a 3/4 race at Hog Hill a week or so later. I didnt look "fat" at 73kg and I dropped 5kg without effort
Likely very linked to body type and metabolism, but if I can do that as a middling club rider, I dont see it as a stretch for a pro to drop 6kg and add power over a longer period