Ulland received an expert diploma at level 6, it seems, but the result is still bad...
A diplomat doesn't give anything here. The land around is "penalized." Ideally, pathfinding + logistics (related to diplomacy) should have been found.
Added 1 minute ago
For the report, you can upload a save before joining Gargonn here. I'd like to see what they can surrender on the second week. My army composition was like this on the third week during scouting:
Snakes Up = 18
Basilisks Up = 17
Hydras = 4
Pegasi Up = 36
Wyverns Up = 8
Sorcerer = 1
I tried removing one slot (except for the Sorcerer) - no luck - the bastards didn't surrender :)
Before Gargonn, you still needed to recruit Wyvern Monarchs - they are more "amenable."