According to the article, Miyamoto wasn't necessarily saying that MMOs would be boring to play (though I think he was indirectly implying that as well), he was saying that they would be boring to make. Every MMO needs constant maintenance and updates over at least a few years to keep the players engaged over that time. Miyamoto, in contrast, prefers to finish up a single project in one go and then move on the next one.

It's true, every MMORPG is a massive grindfest.