I tried to calculate how large an area this is. Assuming a mil is 2000 amot, by the most stringent opinion that would be not more than 4000 feet. Therefore 12 mil would be 48,000 feet, or 9.09 miles. 12 mil by 12 mil would therefore be 82.64 square miles. The entirety of Brooklyn is 96.9 square miles, with 2.5 million people. So by that logic Rav Moshe would permit an eruv in Brooklyn. Manhattan has far fewer people.
What am I missing?