As the story goes, you don't need to be a bodybuilder to write about bodybuilding, articles, stories, etc. You just need to be good at covering the story so that others are interested in what you write. What does writing a story about a competitor have to do with training in the gym for two hours a day? So you can understand it?
Most of the writers, at least a great many of them, as spent countless hours in the gyms in the past. They know what it is like to work out, spend hours doing cardio, etc. So if not all of them are in shape at this very moment (myself included), it doesnt mean that at one time, we weren't.
They were hired to to the job of writing because they are good. It is that simple.
I would love you to ask the sports writer at various magazines how they can cover baseball, football, hockey, etc if they didn't play it.