It's not. AWS goes out of their way to make mixing it with other providers so expensive that no-one will do it unless they absolutely have to, because they are chasing lock-in. CloudFlare basically does the opposite—their whole thing is positioning themselves as an intermediary so they can snoop on and get a cut of the action on everyone "else's" traffic... and then cut everyone else out completely, as they bring more services to market. They are positioned like a boa constrictor, coiling around all their partners and providers that sit behind them.

They are not mainly a CDN and aren't even particularly interested in competing with other companies that are mainly CDNs, which becomes crystal clear if you ever negotiate enterprise pricing with them. The CDN's just a means to an end.

Nb. despite all that their public-pricing plans are such excellent values (though, beware, last I checked the $200/m one was the only one with any kind of SLA whatsoever, and not an impressive one) that if I were creating a start-up CloudFlare might well be the very first service I signed up for. If you're a small fish it's damn hard to justify not using them. And the coils squeeze a bit tighter....

And GCP has a CDN too. Would you use the GCP CDN in front of AWS, or vice versa? It's fairly common to see Cloudflare CDN (& WAF, etc) used in front of services hosted in AWS, GCP, Azure.

Cloudflare is much more than a CDN.