That’s still header filtering. And it should be X-CF- by the way.
(Also sibling is right, I spaced on X-CF- being a header sent to CF customers’ servers. I don’t used cloudflare but cloudfront does the exact same thing)
The whole point is that a genuine user header should never conflict with a Cloudflare internal header, and a browser should never see an internal header.
If all internal headers were prefixed with X-CF-, you could strip them all via SIMD that had no knowledge of any specific header. Hell, you could probably do it on the NIC.