I am unconvinced by this argument. After all, the same light flux is hitting your body. Only thing a lens would do is make it focus in different spots than it otherwise would, but then you'd get spots with burns and spots without burns. This is not what I see happening at all.
It does happen, but due to inflammation and the fact that the “lens” wobbles since it is made out of water, you get enough consistency that it’s not noticeable without a detailed inspection.
It’s possibly still noticeable via the naked eye, but you’d have to compare and contrast to notice the slight mottling.
Nobody doubts that a drop of water can act as a kind of lens. The question is then if a large number of very small water-drop-lenses contribute to sunburn.
The argument is that the light flux (the total amount of energy) hitting your body is the same. A lens just focuses the energy of a wide area (the size of the lens) onto a smaller spot, it doesn't add any energy to the equation. If the lenses are kept totally still, you would get a higher flux (more of a sunburn) in the focal point, and less energy (less of a sunburn) around the focal point. If the lenses move around a lot, it all averages out into not having any effect at all.
There's a counterargument to the above in that thread: the droplet-lenses have a larger surface area than the area they're covering, which lends them the ability to gather more light flux than the area they're covering. This seems rather theoretical, though.
