I think parent's point is that "fake news" is a negative externality of the same kind as pollution.

Which it isn't. Pollution is a well-defined term; you can identify particular offending substances or disposal methods by their impact on humans and their environment. There are precise things that can be taxed to discourage their use. "Fake news", on the other hand, is a pretty uniform spectrum, from the most blatant falsehoods to the stuff CNN and NYT write. It's hard to even agree on where to draw the line (personally, I'd draw the line at "journalism should begin and end at stating verifiable facts + opinions on those facts clearly marked as opinion", but that's an extreme/minority position), and there's no way to start quantifying impact.