you're absolutely right, that's why we included this disclaimer in the post:
Important Note: For this data, along with every other post in this series, we're by no means claiming that the key to ranking a site is simply to buy an old domain, nor are we claiming that a new domain can't be ranked. What we're trying to do is provide data that, when used holistically in your SEO campaigns, can give a competitive edge.
We're just trying to add to the list of best practices you can follow when trying to optimize your site for the search engines.