Well, I addressed this very issue in the post immediately above your first post in this thread.
We are not expected to assume anything. But we have to be able to infer that modernisn is a movement and that modernist art is a part of that movement. If modernist art wasn't a part of modernism, then what ism what would modernist art fall under? It would be very odd if modernist art was considered, for example, non-modern. Because any other inference would be bizarre or odd, we have to infer that modernist art is part of modernism.
The rule for interpreting is to always attach a commonsense interpretation given the context of the text. If there are three possible inferences, A, B, and C, but B and C are bizarre, then we are left with A.
Kaplan Teacher in Toronto