npr.org is not run on flask. There are several independent dev teams at NPR: one for npr.org and associated apps like the CMS (where I used to work), one for show production software, one for producing cool standalone apps (where this came from), and one for station-facing apps. I may be forgetting one. They all have their own typical languages: one uses java, scala, and PHP. Another uses drupal. The team that produced this uses python and flask. NPR is diverse. :)