What makes this guy's blog post a more authoritative reference than 1000 other sites, as well as everyone's personal experience of feeling naturally "good" at some tasks and "bad" at others?

From that artcile:

"'Even in our book, we don’t want to call this theory a complete myth, but instead label it as ‘nuanced’."

But read the next two sentences:

> Why? Well, the basic idea behind this theory is that people are different, and maybe you’ve noticed – they really are. People have different interests, different abilities, different moods, etc.

The author isn’t saying that the multiple-intelligence theory is itself valid. Rather, in an educational context, there is a kernel of value in the idea that different students are different. That’s entirely consistent with intelligence being a single thing.