wagtail.io, a Django/Python CMS, lists NASA as one of its users ... but is not mentioned in the article. The article says the migration was from Drupal to WP. Was that just fake news on wagtail.io?
(I recently spoke to some of the people who worked on these sites; may be misremembering!)
I can't speak about wagtail.io specifically, but a lot of "used by..." sections on websites are based on email signups.
For example, if I run a SaaS and someone @nasa.gov signs up to try it out, I can then list NASA as one of my users, even if they just tested it and never put it into production.
It's not all NASA sites - but used by NASA for some sites.
At my work we used mediawiki heavily but only for intranet.