Dialogue and conversation are not the same thing, though they’re related, just in the same way that stress and anxiety are related but not the same. The task of reading comprehension involves being able to track important distinctions between synonyms.

The second is a counterfactual, and it is correctly deployed to help show the difference between a valid argument and a sound argument. Graham is saying that a good liar presents pleasing and valid but unsound arguments, or rather sophistry.

I think your confusion here is from reading comprehension problems.