Their death is either avoidable or it isn't. If it's avoidable they wouldn't be terminally ill and wouldn't be entitled to assistance. If their death isn't avoidable and they face certain death by starvation, I don't see the argument against assistance.
Voluntarily stopping eating and drinking (VSED), which is what the clause you are citing addresses, is not the same thing as anorexia nervosa.
Anorexia nervosa is a mental disorder, not a voluntary decision to stop eating and drinking. People with anorexia do not typically cease intake of fluids. The illness involves distorted beliefs, compulsive behaviours, and impaired decision-making - it's not a simple choice to stop food and water.
This clause offers no protection to people with eating disorders.