Rav Moshe continues that counting is a chumrah that there would be a definite reshus harrabim, and a kulah that in a place where there isn’t enough people to have 600,000 on the streets we would be able to build an eruv. But then he goes on to say how none of this applies to Manhattan and Brooklyn. There we would always assume there is enough to get 600,000 on the streets. Then he goes on to be lenient if there is a larger area. But doesn’t mention the maximum size like he did in the first teshuva.
There is two ways to understand Rashi in Meseches Eruvin. 1) That any city where there resides 600,000 people is always a rsh”r. Or 2) that if people enter the city daily they contribute to this number. Rav Moshe rejects the first reason because the minhag is to build eruvin even in cities with this number. So he goes with the second explanation. And it follows that commuters count toward this number.