After that big war, Perdan lost and had to give up Duchy of Perdan(Perdan city and surrounding lands) to Eponllyn.
'had to give up' implies it was forced on Perdan. Perdan started the war to destroy a realm, it backfired on Perdan, most of EC wanted to destroy Perdan in-exchange, but Eppy stood up for Perdan's survival once Perdan decided to not fight to the last man and actually negotiate. Perdan itself proposed giving up the Duchy of Perdan and all claims on it for all time, which was deemed as ironically fitting within Perdan considering how Perdan had stolen Eponllyn's capital by first getting a duke to secede from Eponllyn to form a Perleone in Al Arab/Ibladesh, and then conquering Al Arab/Ibladesh in a brutal looting campaign, forcing Eponllyn to sign a treaty that permanently gave up our claims on our homelands and forced us out of them. Additionally ironic as Perleone was initially populated by Westmoorians, whose realm/city was also destroyed in Perdan's War to Destroy Eponllyn in Oligarch, and further populated by more Westmoorians, creating a situation of Eponllyn occupying Perdan and Westmoor homelands while Perdan/Westmoor occupied Eponllyn's homeland.
Yeah, Oligarch stood by Eponllyn by giving up Duchy of Perdan lands(due to Duke Jeroen changing allegiance) to Perdan.
And did not Oligarch ask for Krimml city from Sirion in order to halt their war with Sirion to help Eponllyn?
So "nice" of Oligarch till all its neighbors want attack it 
I think Gabanus was referring to diplomatic support previous to those events, when Garin was still around and probably before even Perdan looted Eponllyn. But yes, Oligarch did abandon Eponllyn/Jeroen/Perdan Duchy because Sirion did not give in to Oligarch demanding Krimml to have a ceasefire and unite against Perdan/Vix.
