Both army value and destroyed value doesn't factor in mercenaries. This is a bug. Wam also didn't delete his own wall. It's destroyed by Beasty's cannon.
The 20% additional olive oil from fishing 🎣⛵ ships is pretty big for Byzantines, because fishing ships are so strong already. In terms of total resources, the payback time of Byzantines building 2 fishing ships and a dock only takes 2.84 minutes for them to pay for themselves in net resources when gathering from shore fish (2.55 minutes for deep-water fish). And olive oil is generally better than either food or wood. Plus if the enemy is investing resources into destroying your fishing, that makes the payback time shorter, even if it reduces your long term gains. Though Byzantines are bad at defending the fishing ships, which is pretty sad for the civilisation 😭.