Yes, I hear OOC that they have broken away from Sirion. But as far as Fontan was concerned, they were a Sirionite colony that was carved from Fontanese land. Thus, they are beholden to Sirion. A totally reasonable assumption, I think, for IC.
I can only speak for Rhidhana and not the rest of Fontan's negotiators at the time of the surrender. To her Nivemus is not a Sirion colony, but rather a homeland for the Rancaguan people cleansed of the poison of old King High Guard and his cronies. She still visits Ashforth from time-to-time to trade supplies and is hopeful that in time the northern realms will heal the old wounds and resume their ancient friendship.
