There's always something magical about the naïveté of thinking that technology can solve people. Bill Gates understood business and technology. Terry Pratchett understood people. Gates is literally arguing in favor of filter bubbles here without the clarity to realize that those bubbles go both ways.
I think you are exaggerating the expertise of both men. Gates was successful, but that doesn't mean he had rock-solid knowledge of all aspects of business and technology. Pratchett wrote humorous fiction, but that doesn't make him an expert on sociology.
Pratchett's comment didn't require a lot of insight to come up with—it's just the (essentially reactionary) observation that publishing on the internet is a free-for-all. And Gates correctly observed that ranking systems like HITS (or PageRank) would recreate, online, the way information sources gain authority in the offline world.
The fake news problem has nothing to do with the web's openness—it thrives on FB. Offline, and independent of the web, the narrative Fox pushes is arguably very close to fake news as well. Neither Pratchett nor Gates displayed significant insight into the that kind of political problem.
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.)
