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 appeared 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.