"Isn't it long?"
長い alone without a copula is the informal form and can end a sentence since it is an い-adjective and acts as a verb. です is only present in the given sentence to make the statement more polite. Informal だ is not used with い-adjectives, only nouns and na-adjectives which act like nouns.
Aside from that, as the other response said 2 years ago, that phrasing has a different meaning to the sentence Duo provides.
長いですね is a statement "It is long" with a tag question "isn't it?"
長くないですか is a question "Is it not long?/isn't it long?"
I think that the issue may be that in English "Isn't it long?" and "It's long, isn't it?" can often mean the same thing seeking confirmation that it's long. A yes to either one of those questions in English generally means that it is long. The answer is relative to whether or not it's long regardless of which question you ask.
On the other hand:
長いですね always seeks confirmation that it is long.
長くないですか can seek confirmation that it is not long.
(Please correct me if this isn't correct.)
I believe that a yes to these questions in Japanese can relative to the specific question you ask, so they are not interchangeable as they may be in English. https://www.wasabi-jpn.com/japanese-grammar/japanese-negative-questions-and-how-to-respond/
I had a hard time understanding exactly what is going on in this sentence, and this article seemed to help:
This construction seems to fall into confirming assumptions construction.
The person asking the question assumes a positive answer for the negative question.
スープを飲みます "I am going to eat this soup."
冷たくないですか？ "Isn't it cold?"
はい、 ガスパチョです "Yes, it is gazpacho"
Something like this seems correct. If I am wrong, please tag me and let me know if I am misunderstanding.
It's been a little while since my last Japanese class, but this is what I recall.
A yes in Japanese to 冷たくないですか？can confirm that it is not cold.
On the other hand, a yes in Japanese to 冷たいですか？can confirm that it is cold. はい、冷たいです
This explains that it can either be as I described above -or- confirming assumptions as you described: https://www.wasabi-jpn.com/japanese-grammar/japanese-negative-questions-and-how-to-respond/
I think that I'm going to avoid asking questions in this way because the answer could be ambiguous!