Yeah, that's why the title is kind of meaningless here. If you did the opposite (make it illegal to build single families in an area with only single families, to promote density), you'd end up with the exact same issue.

A title like "Zoning laws explicitly forbid increasing density" or something would be more interesting/accurate. Saying "X% of houses would be illegal OMGZ!", while true, is not particularly productive as it implies its bad, but the a solution using zoning to increase density could have the exact same headline.