The rock was small enough that it looks like maybe you could build on it. However, I was looking at the snow tileset at the time, which looks different than the others, apparently. http://i.imgur.com/7XWzQJz.jpg

There were a lot of ambiguous cases I found while doing research. I did a lot of testing and screenshotting and found there were times when "tiny rocks" would block building, and other times they would not. I decided not to change them unless they looked obviously wrong. 

I made an exception in this case because the "rock" looked like tiny debris in the snow tileset. I also thought it was reasonable because it maintained symmetry with this template's mirror image (39:s27). (But a map-creator could change the tile manually if they wanted).

If you want to make it unpassable/unbuildable anyway, I don't object.