Mark twain's comment relies on the sarcastic use of tautology:
needless repetition of an idea, statement, or word.
The author Dan Brown mistakenly makes a lot of tautological statements, parodied in this descriptionthis description:
The critics said his writing was clumsy, ungrammatical, repetitive and repetitive.
The last word is an unnecessary repetition of repetitive - a tautology.
What Mark Twain is saying is that to say suppose someone is an idiot and suppose someone is a congresman is a tautology: they are the same thing. (All Congressmen by his definition were idiots.) To say both is a tautology.
To be honest, the writing is poor in the section you quote, because Richard is basically saying that Mark Twain would have considered F. an idiot, which is not something Twain would have been proud of. So Richard either is insulting his wife, or he is really not making sense here, which might be why it's confusing.
It's just a way for the author to sneak in a brilliant quote without having to work for it. It's an example of sloppy writing.
