But an ultimatum is defined as
a demand whose fulfillment is requested in a specified period of time and which is backed up by a threat to be followed through in case of noncompliance. An ultimatum is generally the final demand in a series of requests. As such, the time allotted is usually short, and the request is understood not to be open to further negotiation.The ultimatum belongs to the maker. You cannot reject the ultimatum. You can decline to fulfill the demand.
Perhaps by "rejection of an ultimatum" it is meant - to rephrase from above - you understand the final demand is not the final demand; that it is open to further negotiation.
As the Rev. Jan Nunley presciently stated, "it's not an ultimatum unless you think it is." Perhaps she had inside information about the mind of the house.