Earlier quoted context omitted.
Note that the "contributor" (Mark Nottingham) is also the author of the document in OP, and deserves a lot of credit for taking this approach. Given that his area of work is HTTP standards, I think he initially started removing 418 to tidy up and make things compliant and isn't just some bore who hates fun. Now he's managed to make everyone happy in a standards-compliant way.
It's weird though. The origin spec is specifically a discrete protocol called HTCPCP, not HTTP, so it has nothing to do with HTTP standards. They can do whatever they want with 418 in HTTP.
It's clear that a lot of real-world HTTP implementations are using HTTP 418 to mean I'm A Teapot, whether or not they should, and that reassigning it would cause practical difficulties.