Student or Learner
It think it should be either...not or neither .... is because neither and not are double negative which gives positive but with the given context she meant negative.Bear in mind that neither "The car on the hill which is hard to climb" nor "The car on the hill that is hard to climb" is not a complete sentence unless it is the answer to a question which probably begins "Which car ...?"
It was the reply of one of the top contributor of this website to the thread https://www.usingenglish.com/forum/a...roper-use.html.
P.s. I would have asked this question in that thread itself but the thread was closed.
You are correct.
That particular member makes many fewer typos than Barb or I, but she made one there.
ps. I don't know why that other thread was closed. Ihave re-opened it.