I don't get it either. Do they mean "They (things in the vi- class) are difficult"? I know "vigumu" can be used as an adverb, but does that apply here?
In the last lesson I got a sentence wrong for writing 'difficult' instead of hard and now I write hard and get it wrong because the answer is 'difficult' There are so many inconsistencies in the translations that it can truly frustrate us
Just report that your answer should have been accepted and they will (eventually) add it.