Keep Enemy Closer
No sane king would put the whole nation at risk for another person. Unless that other person is the love of their life or family.
Arihan couldn't distinguish Agost's relationship with Zand and its king. Was Zand fearing Agost will attack them and destroy the Zand if they don't listen to him?
Why don't they fear Anka?
Anka was well known for its powerful mages and never losing the battle against anyone. Even Crisal, the massive kingdom, was afraid to provoke them.
Probably Arihan knew the response. Anka prefers to maintain peace and avoid bloodshed. So Zand must be overconfident that they won't enter their kingdom.
Arihan didn't want to make any assumption but he seemed to guess the reason behind this battle. Keeping him away from the palace and Alora.