Thinker,
If I remember correct, It seems from Rav Moshe that he’s implying to say that a צורת הפתח can serve as a barrier, hence why cities with over 600k had eruvin, as the very area of the Eruv did not include 600k.
So if the eruv i.e. tuzuras hapesech would surround any area with less than 600k, it will be permitted, as we will not be applyingg the rest of the city.