So, in weakening questions, the author has left his analysis/theory incomplete. With that incomplete theory, he goes on to make a strong conclusion. Hence, there is no reason to believe that the conclusion is supported by the theory.
To simplify: (-a -> b) -> c, think of the expression (-a -> b) as y. That is, y=(-a -> b). Then, we get
y -> c,
or, -c -> -y (taking contrapositive)
or, -c -> (-a -> b)
or, -c AND -a -> b