That's fine. By classic I am referring to Tom Landry's 4-3 inside and outside. These use the MLB to cover two gaps, the two B gaps on the 4-3 inside and the two A gaps on the 4-3 outside.
This is all documented, btw, by Vince Lombardi, in the book "Vince Lombardi on Football". He took Tom's defenses with him to Green Bay.
But to summarize, one gap defenses may have a 2 gap defender, just not on the line.
By the time you get to the Miami 4-3 they have this concept of defenses that are not "gap sound". In essence some of the gaps are not assigned and the offside linebackers are supposed to
clean up. Most of the articles describing these ideas tend to be over my head.