But now I think I didn't read the RFC carefully enough, because it appears to be at odds with the Overview on the subject of :reader/:writer on attributes. The Overview says under Helper Attributes: ...

The wiki is out of date. Specifically, the front page of the repo (the actual RFC part) states:

Anything in the Wiki should be considered "rough drafts."

I've now updated the wiki pages with this text. Sorry for the confusion!