According to "https://wiki.openstreetmap.org/wiki/Sidewalks" there are two ways to map a sidewalk. 
Firstly, as a tag on a highway
Secondly, as a separate footway
I was surprised to see that the method used in the center of Athens is the second one. If this is the right way of doing it, there is a lot work to be done.
Still the tag of the sidewalk is wrong since it is marked as a pedestrian street. The right methodology would be tagging it as highway=footway + footway=sidewalk. In addition, the associated street/highway should also be tagged with sidewalk=separate.
I made those changes.
Still, it's problematic that the streets in the square are tagged with having separate sidewalks but they do in only one side of them.