You're correct that dominating a single market is bad, but using your dominance of a single market to bankroll taking over other markets is pretty nasty and you can't trivially do something like that if you aren't already fabulously wealthy from dominating a market.
In the case of the article, it's specifically talking about how Google is doing that.
The problems don't occur when companies are in multiple markets, arguable that is better for competition and hence the person on the street. The problem is when a company or two dominates a single market.