“. I thought it had something to do with ראשנים who hold that a צורת הפתח works on more than ten amos, unlike the רמב”ם and מחבר.”
FYI – The above does not apply wherein you have a status of עומ”ר על הפרוץ.
Also, according to most is only a deRabanon – NOT a de’O’raisa, as posted earlier.
Also, see the below posts regarding 600K..
in any case, Rav Moshe also agreed that 600K according to the Shu”A is not dependent in the city, as his final ruling was to apply it by 12 MIL etc. not simply by the city..