I agree with you. Your mother made her wishes known -- and they were not to die (directly) but to directly refuse a specfic treatment. That same logic would, imo, apply even to those who wished tyo starve themselves to teh point of death. With each hour such a person refuses -- directly, clearly and him or herself, not through proxy or prior contract -- refuses to eat. Food is offered, and they refuse it.
And even then the person should be encouraged to eat -- and in cases where one is dealing with situations of stubborness rather tthan impairment and near-death -- force fed.