This gezeirah does not pertain to us as well for the following reasons:
1. Rav Moshe only stated this gezeirah because he was led to believe that Brooklyn did not have mechitzos. Once we have established that Brooklyn is circumscribed by mechitzos, Rav Moshe would certainly not have objected because of a gezeirah since in any case with mechitzos Brooklyn is considered, me’d’Oraysa, a reshus hayachid.
2. Since we have erected a tzuras hapesach in each neighborhood, the area enclosed contains a population of much fewer than shishim ribo, which would definitely not be mistaken as a reshus harabbim. In fact, Rav Moshe allowed the separation of a neighborhood with a tzuras hapesach (see ibid., 5:28:5).
It is interesting to note the similarities between the Brooklyn and Chicago eruvin. Both Brooklyn and Chicago have a population of approximately 2,500,000 people over a twelve mil by twelve mil area. Despite this number of people Rav Dovid Feinstein shlita was cited in the Chicago eruv pamphlet (West Rogers Park Eruv, 1993 p. 23) as being in agreement that, according to his father, an eruv could be established in Chicago. This gezeirah was not considered a serious enough issue to impede the construction of eruvin in Chicago and should not interfere with an eruv in Brooklyn as well.