On the Los Angeles eruv website it states that Rav Gustman zt"l was asked, “How did Warsaw continue to keep their Eruv, even after the population there grew, and exceeded 600,000. He answered that Warsaw did not have main streets like Paris or Brooklyn, wherein there were 600,000 people from all around using those streets. Warsaw expanded in a manner that the people from the various parts of the city did not use any single streets, and rather each neighborhood used the adjacent streets. At the time there were no cars or wide streets, hence no major thoroughfares used by 600,000 people.”
I then answered:
“This story in the name of Rav Gustman zt”l is not credible. The fact is that Rav Shlomo Dovid Kahane zt”l, one of the main rabbanim of Warsaw before World War II, wrote (Divrei Menachem, O.C. vol. 2, pp. 42-43) that in Warsaw they didn’t want to rely on the criterion of shishim ribo at all (however, he maintained that they relied on the universally accepted criterion of mefulash u’mechuvanim m’shaar l’shaar).”
In fact the most uncontrovertibly proof that Rav Gustman did not utter the above words is the reality that he is the one who found this letter from Rav Shlomo Dovid Kahane and gave it to Rav Menachem Kasher zt”l.
This is what Rav Kasher relates regarding how he chanced upon this letter of Rav Kahane (Divrei Menachem, O.C. vol. 2, p. 42 note 4):
Clearly Rav Gustman knew that the heter being relied on in Warsaw was mefulash u’mechuvanim m’shaar l’shaar and not the criterion of shishim ribo at all.
מענינים מאד המסבות איך המכתב הזה נתגלה לנו. לפני שבוע דברתי עם הגר"י גוסטמן שליט"א ר"מ ישיבת נצח ישראל בברוקלין, שהי' אחד מחברי הבית דין בווילנא שבראשו עמד הגרח"ע זצ"ל