While Nintendo games are good and suited for adults too you can't deny that Nintendo games(and Nintendo's image and policies in general) are aimed to children(well Metroid and something else apart i guess).
The silent implication that you have to make M-rated games in order to appeal to a mature audience is indeed fallacious. The Metroid Prime series and games like Zelda:OoT are good examples. Matureness in games doesn't come with blood, gore and violence.
In any case, I don't think that Nintendo has an inherent image that is repulsive to the "hardcore gamer" and that they cannot possibly shake off. If Nintendo had all the thrid-party games that Sony and Microsoft have, the image of their first-party games would not matter.
They would "just" have to make a console that doesn't look like a toy (the Wii U looks pretty good in that department) and "do a PS4-Sony", like the article suggested, instead of releasing a hopelessly underpowered platform and give western developers the cold shoulder.