yeah, the person you're referring to is confused because the Cloudflare HTTP service terminates TLS and presents a Cloudflare certificate, but that doesn't have anything to do at all with Signal's E2EE which is not based on HTTPS PKI

Last time I used Cloudflare I think their settings default to only "Origin SSL/TLS" (or whatever they call it), which wouldn't encrypt anything between Cloudflare and the origin, it would only encrypt data between Cloudflare and the end-user/browser.