I think Merrith just means in an all encompassing way. Rickon may be the last male Stark that the world knows about, (And really, doesn't the world at large think both Bran and Rickon are dead?) but most of the Stark players are out of the game currently. The house has been dismantled, and other than the family giving some legitimacy to who currently could own Winterfell, finding a living heir is not exactly going to fuck anyone over.
Ramsay wants all Starks dead because they're a contention for Winterfell, and it's assumed the North would ride in support of the Starks. But as far as the other families are concerned? The Starks are a non entity. Winterfell is so far removed from Dorne, King's Landing etc, as to be inconsequential.
The North remembers, but the last rebellion got beat down pretty quickly in a single night.