Nearly every example of a "savant" also shows proportional decline in other cognitive areas. So there's not really much validity in the idea that if we could magically become savants we'd retain our normal range of function. Furthermore, as the article rightly points out, we use different areas of the brain according to requirements. At no time are we using 100%, not least because some parts of the brain suppress other parts according to circumstances (for example, in the classic "fight or flight" reaction, the hippocampus suppresses activity in the prefrontal cortex).
But the idea that we all have untapped potential is both true and false. Most people rarely use their brains for anything more than ballast to stop their heads bobbing around when they walk. Few bother to learn new skills, either cognitive or procedural. Few US citizens read anything more demanding than the TV guide after completing their formal education. Most people are profoundly ignorant of practically everything, because they'd rather gawp at streaming video entertainments. So yes, in theory people could become less dull-witted; but no, in practice there are no magic formulae and practically no motivations for most people to become less cognitively limited. Many even regard being ignorant and stupid as badges of honor - just talk to any populist/nationalist supporting person to verify this statement. This is why the idea that we only use 10% of our brains seems so powerful: when we look at how most people behave and listen to what they say, it is extremely difficult to imagine they are using more than a tiny fraction of their theoretical maximum.