Two things.
It's possible to be great at something simply by practicing, assuming normal capabilities. But great here just means "better than virtually everyone". Being mediocre among people who practice regularly it makes you immediately better than basically everyone who has done it once or twice. By most definitions that's "great".
Median daily StarCraft ranked player? You're great at StarCraft.
Second, if you start young enough, you get the compounding effects of time. You're now "pretty good among lifelong daily players in their prime". That's Olympic/ world class.
Like that guy who had kids just to make them Chess masters. He did so by making chess part of the family life, so integral it wasn't working it just was. The guy from the original post actually.
So it's tempting to say things like TFA posits, and while I'm not sure it's 100% true, it's definitely not 100% false or pure rubbish.
I think the article itself mixes these concepts. It starts with the world-class level example, and then jumps into learning new skills over a week.
In order to be great, indeed, you don't need to be obsessed, and very rarely you cannot achieve it -- you just need to be relatively consistent. And I 100% agree that if you start early, maintaining a skill on the "better than vast majority" level is not too hard (the hard work was done in the past).
But to achieve truly outstanding results, you definitely need to dedicate tons of time and ideally be obsessed with it (at least very-very interested).