There's a lot of detailed discussion about different types of cells. Vero cells don't have TMPRSS2, but Calu-3 (human lung epithelial) cells do. To dramatically oversimplify, this compound works on the latter type but not the former.
They also did a little safety testing in a mouse model. This is early stage, as you point out there are a lot of things that work well in the lab and not so much in real humans, but still I find it promising.