instructs:
brew install -cloudflare
That absolutely isn't correct. There is no such homebrew formula.
points to the above page, so it's no help at all.
Using 'brew search' finds no plausible homebrew fomulae or casks for the cloudflare plugin.
What am I missing?
Note that installing the whole business via MacPorts, which is not even mentioned in the documentation I cited, works perfectly fine.
You may get a reply from some other volunteer with MacOS experience.
But, your concerns are better addressed to the EFF who develop and maintain Certbot. Their github is: GitHub - certbot/certbot: Certbot is EFF's tool to obtain certs from Let's Encrypt and (optionally) auto-enable HTTPS on your server. It can also act as a client for any other CA that uses the ACME protocol.
OK. I will merely observe here that I got here by following the link on the page where I found the documentation I was trying to follow. If this isn't the best place to seek assistance with those things, perhaps this forum shouldn't be linked there :-). I'll point that out over there as well.
Sure, I understand. While running Certbot it also refers to here for any problem even when using it with a Certificate Authority other than Let's Encrypt 
Plenty of people come here with problems using a variety of tools. And, many times we can resolve the problem even if it is not a direct problem with Let's Encrypt (it rarely is). It is not always easy to determine what the actual problem is. If we can't resolve it we give a good suggestion for the best next step.
You are welcome to ask questions or post concerns here. In this particular case I suggested contacting the EFF because they would need to update the docs.