It's not enough for the servers to support it, the resolver has to actually use it too. Your guide instructs people to configure the Cloudflare / Google DNS in system preferences, which will query those via plaintext DNS on port 53.

GP's point is that if you want to secure DNS, it's a bit more involved, hence the build in "instructing people to build and configure a HTTPS resolver".

I don't actually use this, as my home router handles DNS over TLS for everything, but a quick google search turns this up: https://blog.because-security.com/t/use-cloudflare-dns-with-....