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.
[0] https://www.scientificamerican.com/article/the-magnifying-ef...