It's more like Cloudflare forked nginx a long time ago, and is meanwhile in the very slow (like, decade-long) process of replacing it entirely.

The Cloudflare Workers Runtime∗, for instance, is built directly around V8; it does not use nginx or any other existing web server stack. Many new features of Cloudflare are in turn built on Workers, and much of the old stack build on nginx is gradually being migrated to Workers. https://workers.dev https://github.com/cloudflare/workerd

In another part of the stack, there is Pingora, another built-from-scratch web server focused on high-performance proxying and caching: https://blog.cloudflare.com/how-we-built-pingora-the-proxy-t...

Even when using nginx, Cloudflare has rewritten or added big chunks of code, such as implementing HTTP/3: https://github.com/cloudflare/quiche And of course there is a ton of business logic written in Lua on top of that nginx base.

Though arguably, Cloudflare's biggest piece of magic is the layer 3 network. It's so magical that people don't even think about it, it just works. Seamlessly balancing traffic across hundreds of locations without even varying IP addresses is, well, not easy.

I could go on... automatic SSL certificate provisioning? DDoS protection? etc. These aren't nginx features.

So while Cloudflare may have gotten started being more-or-less nginx-as-a-service I don't think you can really call it that anymore.

∗ I'm the tech lead for Cloudflare Workers.