Are you implying that Etsy's engineers aren't good?
More to the point: The idea is that if you're a company working with a high-scale website, don't bog yourself down with PHP if (a) you don't have to and (b) it doesn't fit your organization.
If you're planning on working with PHP at scale, you have to be hacking the codebase extensively. Yahoo used PHP back in the 90s, but they had so many C extensions that the PHP was effectively thin glue.