they do use it in their speedtest server.

  curl -v https://speedtest.ams.t-mobile.nl.prod.hosts.ooklaserver.net:8080
  ...
  * Connected to speedtest.ams.t-mobile.nl.prod.hosts.ooklaserver.net (2a02:4240::e) port 8080

Probably a requirement from Ookla, so again "They refuse to implement anything that isn't strictly required".