As per this article: https://www.nature.com/articles/s41598-020-67211-2

> ... far-UVC light has a range in biological materials of less than a few micrometers, and thus it cannot reach living human cells in the skin or eyes, being absorbed in the skin stratum corneum or the ocular tear layer.

The ocular tear layer, according to 30s of googling, is just the wet layer over your eyes (the stuff that turns into tears when you produce to much of it). So the idea is that this is eye-safe because the radiation gets absorbed by the small layer of wetness on your eyes.