Andrew Cavanagh’s “origin story” was pitifully inept. Yet his audience swallowed it whole.

There is nothing quite so tiresome—even for the political obsessive and the geek—as the obviously contrived origin story. As one of the aforementioned political obsessives and geeks, I’ve seen the origin story in all of its variations. Some true. Some too good to be true. Some partly sensationalised. Others entirely fictionalised...