It’s not true that an eruv doesn’t work in reshus horabim. If it were true then you couldn’t carry in your house!
What is true is that, MID’RABBONON, an “eruv” consisting only of four poles and four strings doesn’t work in reshus horabim. You need more than that. How much more is not clear, but certainly if the majority of three sides is made of solid mechitzos, and any road that is reshus horabim has doors installed that can close it off, then is effective even mid’rabonon.
DaMoshe, the requirement (according to those rishonim who hold there is such a requirement in the first place) is clear: the 600,000 must pass through BECHOL YOM. Every single day, not once a year.
R Moshe was very misinformed about Brooklyn’s population. He originally wrote with certainty that Borough Park and Flatbush alone have 600K each, and the whole Brooklyn has more than 3 million. This wasn’t the case. This is just one of *many* instances where his knowledge of the metzius was incorrect, because he was misinformed by those on whom he relied for such information.
As for where the whole 600K thing comes from, it is a mystery; we can only assume there was a girsa of the gemara that the Ashekenazi rishonim had, which did list such a requirement. (A girsa has been found that says there is no reshus horabim in Bovel, but not why.) But be it as it may, the overwhelming majority of rishonim and geonim held this way, so it’s difficult to argue that the halocho should not follow them. The most we can say is that someone who wants to be mehader bemitzvos should be machmir like the minority of rishonim who held there is no such requirement.