Is it true that about 95% of firms report constant or falling marginal costs, which undermines the premise of a rising supply curve?

According to Steve Keen, which I know this sub takes issue with, orthodox theory which posits marginal costs should be rising. But Keen cites empirical studied that show about 89% to 99% of firms have constant or falling marginal costs, which means that the orthodox labor supply/demand curve is fundamentally flawed. He goes on to say that the actual labor supply is not a smooth, upward sloping curve, and that workers are not paid their marginal product.