Simply because the players aren't told all of the history of the WLD, doesn't mean that the history doesn't inform play through the DM's treatment of the material.
Same thing about writing a story for readers, actually.
When you write fiction, you can engage in acts of worldbuilding that will not show up on the page directly, but will influence your treatment of the story the same way the treatment of the story influences the worldbuidling occuring
de facto.
Let's say I'm writing a scene taking place in heaven. I need to find some element to evoke the feeling of heaven without forcing tangible descriptions on the reader. I happen to look by the window at that moment and I see the sun rising. I decide to use this as an image in my writing: the character arrives in heaven, and the sun rises in front of him. I describe the color, the warmth, the fresh scents all around, the feeling of spring and rebirth, so on, so forth.
Point is, people won't know I've been looking at the sunrise while writing this. They don't need to know in order to understand the story.
The same way, cultural materials, history, ethnicities etc I would create to tell a story won't necessarily appear directly in the story, but they will influence my descriptions of people, places etc in the story itself. They will affect my own familiarity with the elements of the fiction I'm writing. Sometimes for the better (depth, coherence, verisimilitude), sometimes for the worse (needless side-tracking, confusion of the reader, boredom). It all depends how you choose to use the tool.
The way Harrison is wording it, worldbuilding's always bad. It's dull. Unnecessary, an ENEMY of writing (and before there is any criticism of this, please read his statement again: he's not saying that "may" or "may not" be bad, that it "may be excessive in some instances". He's saying that's the antagonist of good writing. Period). It's just so shortsighted it's laughable.
There had to be an agenda behind this if we make the assumption the guy actually has brains. Looking at
today's entry on his blog, it's clear to me there was.