I doubt there are over 47 sq. mi. of vacant lots in the city, that would equate to over 225,000 individual 1/8 acre lots. There is no way that much acreage sits empty. Since 1969 there have been about 175,000 demolition permits issued to the city. However that doesn't just include individual houses. It includes major apartment buildings, housing projects, etc. that would otherwise fit more units per 1/8 acre. It also doesn't take into account the fact that many of those houses were demolished to make way for new developments, and aren't necessarily "empty lots" today. Nor does it take into account all of the new infill that has gone up throughout the city.
I don't know, I guess I would need proof to actually believe something like that.