On top of that, most potential employees are shit at negotiating. Most never negotiate at all - they take the offered number or leave it, but they rarely push.
This is why the article sounded so strange to me. It seems like the idea that's really being pushed is that negotiation is bad, which is an easy sell because most people have a strong aversion to it. They're eager to find a reason to not have to do it, even when they suspect they're underpaid.
It's the uncertainty that causes so much angst, not the money in absolute terms, and appeasing that uncertainty by convincing the median earners that it's okay to gossip seems like a great way to get them to never ask for more. They will gladly continue leaving money on the table if only they can be confident that nobody else is taking it.