I think the teacher had a point. It is highly possible that the lead instrument thing was all but a distractor.
Closely look at the statement: Guitar was used in rock and roll from the 1940s. The statement implies 2 aspects:
- Firstly, rock and roll first appearedhas been around from the 1940s.
- Secondly, guitar was available for use in rock and roll by that same period of time.
The 1st line of the passage mentioned: In the earliest rock and roll styles of the late 1940s and early 1950s
- It can be drawn from the passage that rock and roll didn't exist from the 1940s until late 1940s.
Probably that's what make the statement false, not the lead instrument thing.