In particular, you referred to paragraph (a) and the wording “serious and incurable”. Paragraph (c) makes it clear that the suffering is intolerable and cannot be relieved under conditions that they consider acceptable, which is a slightly different formulation than the one used by the Supreme Court.
However, it actually tracks quite closely with the formulation of the court that an individual may have, as you say, a condition that is otherwise medically curable. Some people may choose to have treatment that's almost unimaginable in the grief and suffering it causes them, but will continue with that treatment. Others may find that the treatment that is available to them causes them suffering that is intolerable, and by paragraph (c), it's clear that it's their choice and the law does not in any way impair or affect that choice.