We have already seen the map, and there are many more than 5 cities. There are around 23-25 multicell settlements/major locations scattered across the map. Pete didn't say anything about the 5 major cities and he didn't even indicate the screen was a city, hecwas just asked how large the cities would be and he suggested looking at Makarth and thinking for yourself. Basically, he means that the cities will be way more impressive, dynamic, and interesting. Even if that's just ruins, it tells us how the locations are better and if it's just a ruin it's actually better to hear since if that's how ruins look the major cities will be way more impressive.
We know it's not a major city, because Makarth Side is not one. We don't know if it's Makarth, the ruins, or Makarth Side, the settlement. Or if Makarth Side is ruins or Makarth is a settlement. We don't know much other than whet they already told us.
We do know that they already said the Makarth pic is ruins. He's just insinuating that because those ruins of a Nordic settlement are so impressive and dynamically interesting compared to Oblivion that the still inhabited and recently constructed Nordic settlements themselves will be similarly impressive (as seen in sick concept art).
There is no reason to release a screenshot of an unpopulated city when even towns that weren't completely traced into the map we've seen have been demoed to the press and screenscapped. Let's not be silly over a vague comment. It won't even surprise me if he throws in a group of bandits/hermits/goblins/archeologists/plane crash survivors at the last minute to mess with us. It would explain how the roof looks so good if it's a badly durable form of tile, though.
The thing about Twitter is that nothing on there is a press release or set in stone, they can give details they normally wouldn't give just as they can be vague or misleading to put us into hissy fits.