That is not what they do, though. Just because a non standard configuration is possible doesn’t mean that’s the best thing to use. DNS, by design, uses authoritative nameservers, which is what cloudflare and quad9 host. These authoritative hosts distribute their records to caches (usually just recursive DNS resolvers) to ease and distribute the load. It’s literally in all of their documentation, and explained in pretty plain english on their pages.
Much of the Quad9 platform is hosted on infrastructure that supports authoritative DNS for approximately one-fifth of the world’s top-level domains, two root nameservers, and which sees billions of requests per day.
When a record is updated in your domain (or cloud) provider, it is distributed via an authoritative nameserver hosted by that company. These get distributed to the root name servers, which then distribute the records to other authoritative nameservers.
I don’t know why you’re arguing over this, when it’s one of the first things you learn in information systems and networking. Sure, there’s a lot of stuff for the infrastructure. But the way DNS works on these hosts is still the same, and blocking a single record is not difficult and does not take extra engineering effort. The authoratative hosts simply change their records and it’s done. DNS takes care of the rest.