Sure, but in practice most people probably use Cloudflare with Nginx/Apache. They are more like complements, rather than substitutes.
They don’t use nginx anymore
Whether or not Cloudflare themselves use Nginx any more is quite beside the point. My point was that most people in the world using Cloudflare proxy service don't use it on its own, people still use a webserver like Nginx/Apache behind it, so the original tweet doesn't really make sense.