That monument is a grave which has the religious orientation of those memorialized marked as atheist. It is no more an atheist monument than a grave of a shoemaker is a monument to shoes.
So no matter what kind of monument could be erected to atheism, you're going to just deny what it is? That's fine I guess. I'm still not sure what the point is - if your contention is true that it is not possible to erect a monument to atheism, what does that imply? I don't see that it's really that meaningful, even if you were right.