Dear Youdont,
Rav Moshe’s shitah is clear and constant: Six hundred thousand people on a street makes that street a reshus harrabim. The same number out and about in a city, makes the public places of the city into a reshus harrabim. Precisely how to count a city is of no consequence, because if it is close than we are unable to guarantee that the eruv will contribute to shemiras Shabbos and are exempt from building one.
Now to our little mudfest:
1) So you are left wondering about Rav Moshe’s intentions with his Flatbush teshuva. Because there is no chance that you are wrong here. But you won’t wonder about it either. We should ask Rav Moshe to wonder for you.
2) Rav Moshe writes that he is forced to write about Flatbush, because people think he permitted it. What compelled him? Is it possible that he believed that people are not allowed to carry with an eruv based on a mistaken belief of Rav Moshe’s permission?
You can split hairs now.
3) Comprehension alert! Without Rav Moshe’s opinion, there is less of a reason to build an eruv in a mega-city. The simple understanding was that a population of 600,000 makes the city into a rsh”r. Rav Moshe is bringing about new ways to be lenient throughout his teshuvos. So take away his chiddush and try to use the Aruch Hashulchan. (Except that the Aruch Hashulchan is trying to incorporate the Mishkanos Yaakov into the accepted halacha. [Meaning, that the sugyos go according to the Mishkanos Yaakov, but the halachah is different because of his chiddush.] But I doubt that matter is a factor to you.) Nobody is adding to Rav Moshe’s opinion to forbid the eruv. And there is no way around it. His shitah is very straightforward, as it appears in chelek aleph.
4) A thourough review of Rav Moshe’s writings, show that many of his rulings were not given with a horaah lasos. Rav Moshe was very humble about forcing his opinion on others.
You can still build any eruv you want. You can even put up an eruv in a bungalow colony, that nobody will check for years on end. You can build an eruv without knowing the topography. I don’t care. But don’t go around proclaiming that Rav Moshe’s shittah is not definitive, when Rav Moshe clearly says it is. As he told Reb Tuvya, you can disagree with him. But at least make the effort to properly understand the teshuva.