Well...didn't they unite under one political faction called An-Saxhleel in order to push back the Daedric invasion?
The wikis state the unified Argonia was so powerful,that they were able to scare the Daedra off their province.
After that, they finally had their revenge on the Dunmer.
But that was it, Beth didn't say anything further about Black Marsh in Skyrim...maybe that's why Guard said "supposedly", since there's no way to know how they're faring off at the moment.