And I think you're being a little unfair to Terry. Sure, he wrote humorous fiction, but his grasp of human behavior is the source of much of the observational humor of his books. They are, essentially, spoofs of society.
This is like the argument that comedians who make funny jokes about human nature must therefore have good insight into politics. Anyone who watches comedy knows that the ability to be funny doesn't equate to having a good analysis of the situation.
You assume his understanding of society is accurate because it is popular? Or because he is popular? You can just as well assume Bill Gates understands society for exactly the same reasons. (Gate's business results are not even a spoof of reality.)