Cloudflare is a data goldmine setup by people who love fedoras and newspapers.
Professional DDOS providers won't use Cloudflare ever and have the skills, metal and (human) network to do everything in-house.
Zero ingress puts the upfront bandwidth cost onto the attacker. Because... you actually may succeed to defend and stay up. Their success is not guaranteed, they might be shouting into the void.
Attack success (as in, "impact on you") is guaranteed if your ingress is chargeable.