GAON:”That’s is exactly my point, that at the end of the day, Rav Ahron is NOT in accordance with the MB, (Despite the MB being machmir). And his understanding of the Rambam is certainly not like the Shu”A and the Magid Mishnah.”
Rav Aharon has many chiddusim in his teshuvah regarding eruivn, including his understanding of the Bais Ephraim and mefulash (in which case he is arguing against the Magen Avraham and many poskim).
“It is also worthy to note, that Mishkenos Yaakov himself acknowledges that his shitah is not the Minhag, even in Karlin (town where he was Rav) they did not abide to his shitahs (I recall a responsum in Zkan Aharon of Karlin permitting delosos like the Rambam in Shu”a, actually I think its components were based on relying on the most lenient shitos…):
There is also the fact that there was an eruv in Karlin in the time of Harav Dovid Friedman. I think the teshuvah is in Zkan Aharon 1:21.