That seems vaguely accurate. However I would suspect it’s significantly more.
The things it doesn’t cover were lost trade and damaged production.
The loss in 2000 Slaves sounds like a rather awful effect upon ones means of production. I mean I suspect the Grendel now have their own “Winds of Fortune” options where they can either just raid*, or force lesser Grendel Vassal tribes into the mines but that would result in fighting and further damage.
The other will be impact upon trade - each territory provides taxation, this would be heavily impacted by the structural damage and the political damage or the ports no longer being safe. So they’ve lost money through there that takes time to build back up.
All these are I’d say equivalent to some of the nastier economic territory curses. Ofc this is somewhat below the abstraction layer as the Barbarian economic systems while still the same as the Empire (they need taxation, they need supply for armies etc.) are both more simplistic and more “eyeballed”. So an exact figure is impossible.
The other thing to address, is the elephant in the room which is why the opportunity existed (this is information on the wiki). The Grendel had all their armies and all their fleets anchored off Sarvos, this is why their raids were so successful, they were, as per common wiki knowledge, throwing everything they had into the raids after had destroyed a shed load of their armies.
If they don’t want it to happen again they’ll have to send some (no idea how many) Navy’s and Armies back to ensure their coastline is kept safe.
Hell they might back off completely focusing on maintaining their trade as that’s their bread and butter. Or they could continue raiding in a smaller scale, who knows really.