iacisrmma
Rav Moshe does agree that there are shitos and rabbanim etc that you cannot be mocheh.
However, according to many, the facts are that there are three Mechtizos surrounding Brooklyn, as most seawalls are built up with gates etc.
Also, there are psokim and poskim that were relied upon prior to Rav Moshe, like the Eruv in Warsaw which had more then 600K inhabitants.