The following letter that Rav Moshe Feinstein zt”l wrote to Rav Shalom Yehuda Berman of the Lincoln Square Synagogue should put to rest the claim that kol koreis (especially those regarding eruvin) carry any weight at all. The fact that in this correspondence Rav Moshe declared that even in Manhattan ― where he signed the kol korei opposing the eruv[*] ― Rav Shimon Eider zt”l does not need to follow his opinion and can establish an eruv is proof that Rav Moshe never subscribed to the rhetoric of these kol koreis. How much more so regarding Brooklyn where Rav Moshe did not sign the kol koreis at all (see The 1979 Flatbush Kol Korei Exposed and The 1981 Boro Park Kol Korei Exposed) would Rav Moshe have allowed an eruv to be established. [I would add that if Rav Moshe would have known the facts on the ground, he would have even agreed to the eruvin in Brooklyn in their present construction.]
My apologies for the quality of the scan. As of yet, I have not been able to obtain a better copy of this letter.
[*] The only support that Rav Moshe signed this kol korei is the addendum to Igros Moshe, (vol. 6) O.C. 4:89. The fact that this is the only addendum in any volume of Igros Moshe makes it somewhat questionable.