(of a person or their manner) unwilling or unable to believe something: an incredulous gasp.
What I believe is that Frank or McCusker gave Dieterich and asked him to do limited research and offer some advice. Dieterich wrote a brief letter. He didn't deliver a research report or cite sources or the like.
And this is the only advice he gave, as far as the merger transaction was concerned:
You have a completed merger, but you merely need to modify the name of the resultant or surviving entity, which is a simple process that you've already planned for.