The 600k according to Rav Moshe’s chiddush is the number of people living in the city. Most people understand it as the number of people traveling through the street in question.
I think the language seems to be difficult according to Reb Moshe. Shishim Ribu bokekin bo seems to imply movement.
One other question I have in general is why the requirement for lechis on utility poles. Why does the pole itself not serve as the lechi? The nailing of lechis to the poles is what causes all the eruv fights among the non-religious, etc. Any mevinim here who can explain? I know the wire must be directly over the lechi, but doesn’t that usually happen with the pole itself?