So I’ll ask this question with sincerity as you seem to be the scientist in this discussion If you set a sheet of steel next to a block of wood out in direct sunlight, the sheet of steel would get so hot that you couldn’t touch it, while the wood would not, correct? Is it possible that the plastic cover gets hotter than the water can in direct sunlight, then also does not allow that extra heat to escape? Hence, creating a little bit of heat at least?Magnifying glasses do not amplify anything. They concentrate the light from a large surface to a small point. Since a solar cover is the exact same size as a pool there is no opportunity to concentrate light.
Your belief is irrelevant. The laws of physics do not provide for "amplification" of something without the added energy coming from somewhere. Neither you nor Casey have offered a good answer for where that extra energy comes from.