maybe because their series is so refined and the characters are eloquent that it doesn't right with me that annulment was used. for it's exact usage, annulment is used after marriage. it may sound like divorce but the law defines the two as 'ending a valid marriage' and 'recognizing it as a void marriage.'
from what I understand while in divorce you may call your spouse your 'ex,' that would not be the case with annulment, with annulment that status will be perceived as if it never happened. it's used in religious practice to not recognize the marriage as if it didn't exist.
annulment examples would be ones not recognized by the law, without legal documentation, like group marriage. and annulment is also a desirable result from contesting forced marriages as well. divorce is on terms agreed by the two parties involved, usually to split properties and payouts, but annulment can be recognized with only one party if one of them applies. they can also be made if it's believed that one of the parties is 'incompetent' without a guardian to advocate for their decision. last example I can think about is a shotgun wedding done while one of the parties is inebriated, meaning if they can prove they were incapable of making a decision the marriage will not be recognized, or 'annuled'
it's not annulment but a termination of an agreement made by two households. not only was it an agreement made by her family meaning she herself does not have full autonomy for her arrange marriage, it requires his signature as well in order to terminate the contract.
edit: but I reread it and it's Orva who says 'annulment', which is quite fitting because his IQ is pitiless