He already had lots of traction. I think the author's supposing that Zuck went to a VC before FB took off like crazy.
Sure, Facebook may not have made sense until it existed in some form, nor friendfeed, but that doesn't mean they couldn't have a good elevator pitch, the pitch is often showing off part of the product, not just an idea, which by itself is largely worthless. Most VCs don't fund just ideas, there has to be at least some basic implementation already.
What is this point talking to? I don't think it addresses anything in the post, and I think you might have read it incorrectly.
The author's arguing that elevator pitches don't do justice for some products; that a VC essentially won't be able to understand/grasp the power of a product until s/he sits down and uses it, a la the Nintendo Wii or the iPhone.