"Isn't that difficult?"
難しいですか？would be closer to 'Isn't it difficult'.
When this course uses specific pronouns (He, she, I, this, that) it expects direct translations. Otherwise, 'that' becomes 'the' and 'we' becomes an unknown individual, etc.
In spoken Japanese it might be excluded based on context, but with the assumption that the listener understands to what the speaker is referring.
I would say that generally, your exampme is not correct because it implies that you have also tried "that", thought "that" was hard and want to gave the other persons confirmation that "that" is indeed hard.
The ね always imply that you have experience doing it. Think of it as a "right" in "Isn't that hard, right?"