If Rav Moshe clearly assumed a clearly incorrect fact, then presumably we could ignore the psak.
However, I am not aware that any of the 11 reasons which were brought to argue with him at the time, and which he answers in Igros moshe, OC 5:28, were that his facts were wrong.
As it happens, it had to do with the total number of people living in Brooklyn, since he held that if there were usually 600k people on all the city’s streets combined, during times that people are in the streets, then the whole city is a R”H.
But, I am sure this thread has discussed that, and I am no expert on this at all. I just started reading his teshuvos 5 minutes ago.