Tazria-Metzora 2 - 4 Metzora'im

This week's haftara starts out with a setting of four metzora'im sitting at the gate to the city of Shomron. Rashi notes that the reason they were at the gate to the city was as per the prescription in this week's P' Tazria (13:46) that the Metzora is required to dwell outside of the city. According to Keilim 1:7, this law applies to any walled city, to which R"O miBartenura adds that it must have been walled from the times of Yehoshua bin Nun. This being the case, why was Shomron included in the restriction, being that we are told (Melachim I 16:24) that the mountain of Shomron was purchased by Omri (father of Ach'av and grandfather of the current king Yehoram) and built into a city, implying that it did not become a walled city until more than 5 centuries after Yehoshua?

R' Akiva Eiger (Keilim 1:7) notes that Targum Yonasan anticipates this question in his translation of the passuk. Rather than translating "har", mountain, as its usual "tura", he translates it as "k'raka", city. Hence, Omri purchased an already existing city, and merely increased its fortifications and developed it into his capital.