I've never seen the evidence where density increases drive down existing land/home values.

it's not that density per se drives down existing costs, but density almost always brings more housing stock to the market (unless they are simultanously tearing down housing elsewhere) and housing stock drives down the cost of housing, which is the point of the original article.

So if we take it as an assumption that density increases housing stock, there is lots of evidence that density drives down prices of existing land/home values.

in fact, density increases the value, because the original plot can now be resold at a higher price for a hi-rise.