Automattic is running a pull based CDN that would literally work on any URL. Hardly creepy <a href="https://developer.wordpress.com/docs/photon/" rel="nofollow">https://developer.wordpress.com/docs/photon/</a>
It appears that the Automattic CDN does not just fetch and save files from sites without Wordpress or Jetpack, it fetches and saves them even if robots.txt says no crawlers.