I suppose you could go to smaller cities/towns and say that it would also be illegal to build apartments there - but the types of houses in suburban towns built 50 years are still very legal today.
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.
I'm fairly certain you could buy an apartment building in Manhattan, demolish it, and build a picket-fence house there. It was just lose you tens or hundreds of millions of dollars so no one has done it.
From what I can tell the way that interactive was constructed it would also highlight a lot if it fell into the category you mentioned but I can't find any where this is the case. It is a one-side problem and to pretend like it is two-sided is a bit detached from reality.