It seems to me that you have misrepresented the views of Rav Moshe and Rav Menachem Kasher in Insights to the Daf cited below-
>>It is not clear, though, exactly what is considered Mefulash.
(a) RAV MENACHEM KASHER (Torah Sheleimah 15:172) writes that any road that starts at one side of Reshus ha'Yachid and exits at the opposite side, regardless of how much it turns and bends in between, is considered Mefulash.
(b) RAV MOSHE FEINSTEIN (Igros Moshe OC 1:140) writes that a road is considered Mefulash only if it passes straight through the city without bending at all. (This is obviously a much more lenient ruling, since it is very uncommon to find a road that "pierces" straight through a city.)<<
Rav Menchem Kasher is clear that Mefulash, as in the diglei midbar, is when it doesn't bend and he uses this as a kula in Eiruvin.
Rav Moshe however says he is not clear on this issue and therefore did not want to write about it but seems to conclude just the opposite of what you quote from that teshuva. He is not being lenient but being machmir.
I would appreciate if you would enlighten me.
Chaim Zundel Pearlman
1) Rav Moshe Feinstein, in the above teshuva, cites the opinion of his questioner that a city is only considered to be a Reshus HaRabim if the road passes straight through the city without bending at all. The questioner in Igros Moshe wanted to be lenient on the basis of this argument, since it is very uncommon to find such a city nowadays, and therefore it would be unusual to have a Reshus Harabim.
2) When we wrote that this is obviously a much more lenient ruling, we did not mean that this is the conclusion of Igros Moshe, but rather that it is the suggestion made by the questioner.
Rav Moshe, himself, was indeed Machmir; as you write, Rav Chaim Zundel; since Rav Moshe wrote that it was not clear to him that this is the correct pshat in Mefulashim mi'Sha'ar l'Sha'ar and therefore he did not want to rely on the leniency suggested by the questioner.
Chodesh Tov
Dovid Bloom