That is a far from ideal response but I can understand the logic. The battery will be discharged through lack of use, which would affect its capacity and ability to maintain a charge - 12v lead acid batteries do not like being deep discharged. In the eye of the battery manufacturer, this isn't a warranty issue because it isn't a manufacturing defect, it has inadvertently been user damaged.
I'd argue that a vehicle isn't fit for purpose if you cannot go on holiday for two weeks, so you will have some recourse if you wanted to threaten or take up legal action. However, just replacing the battery with a new one doesn't solve the issue, and I doubt there is room to fit one with a larger capacity.