This is about CFs public DNS resolver though, and not every domain they're ordered to stop resolving will also happen to be served though their own CDN. In this case it was, which explains how they're able to serve a 451 error over HTTPS, but that won't always be the case as the article implies.

In some other cases I suppose they could downgrade the connection to HTTP in order to show their 451 page, but if the domain is HSTS'ed then that wouldn't work either. That'd have to just black-hole the query like Google does.