In this example, claim as a reporting verb is used to indicate that you understand that it is Johnson’s opinion, possibly without strong evidence to support it. In a way, you are giving your opinion on Johnson’s argument, indicating how sure you are about it.
Using argue, on the other hand, would make your readers think that you have good reason to believe Johnson’s argument/evidence. In other words, you can show whether you agree or disagree with an author by choosing the right reporting verb.