Its a little bit of a misleading headline. A lot of people ignore their check engine lights these days because they are typically indicating that either a sensor has gone bad or the seal on their gas cap isn’t sealing well. The sensors some time can mess up your fuel economy, and as others have noted plugs that are worn out, fluids that need to be changed, filters that need to be replaced all constitute what most people would consider a tune-up these days.
There was some sitcom years ago where a woman complained that her car had totally stopped running.
Her friend said, “Your check engine light was on, right? Did you get that looked at?”
“No, why should I? It’s still on. The light is fine. It’s the engine that’s got the problem!”
I have read that Oxygen sensors lose their effectiveness over time and though they may not trip a code, that can affect your gas mileage.