Rav Moshe makes some additional side points. These may have come from the in person discussions. And then reiterates why he didn’t stop them even though he disagrees with putting up the eruv. His wording is, ‘that it is against what he holds is the law’.
Rav Moshe Feinstein emphatically giving his opinion was not a game changer for eruvin. A year later, he wrote another teshuvah. It is just his reasons to not build an eiruv in Flatbush.
1) The assumption is that there is 600,000. One would have to prove that there isn’t. And even if they do, people won’t know about their proofs. And similar considerations.
2) The Rashba about public open places (platya). This could be even if the total population is 600,000. There isn’t a clear precedent for this complication.
3) The area can not be measured randomly. It would start at an edge. In Brooklyn’s case this would be the beach and the river. (Then he discusses if the old Coney Island would be a problem since it was only in the summer.) But it would still be the problem of people aren’t aware of the area calculations either.
Then Rav Moshe writes that all three reasons are valid even according to what they told him that there are not 600,000 in Brooklyn. Meaning, that there is no way to build an eruv in Brooklyn even if there is not for sure a rsh”r. It simply runs into too many problems that we don’t have a precedent for. So Rav Moshe held not to put up an eruv in Flatbush and avoid the problems. Which is comparable to Yerushalayim.
Then Rav Moshe mentions the city map that would make Brooklyn a rsh”r with Manhattan. But the river should separate Brooklyn from Manhattan. But still it would be a rsh”r doraisa according to that map.
For anybody who knows what happened next, Rav Moshe did not protest the eruv based on this map.