That's centanly one interpitation. However, the information is being provided by Spock during the mind meld. Calling him "Last of the Romulan Empire" wpuld be like referring to Checkov as "Czar of all the Russias". He may have called himself that but it's not relavent to the point Spock was making.
Seeing as the Hobus Star was a fair distance away from Romulus and still managed to utterly destroy the planet it stands to reason that the otherRomulan colonies, particulary those older and more established ones, would be destroyed as well. How many Romulan colonies have been mentioned before anyway? Perhaps the Romulans have captive planets but very few that they've colonized themselves.