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The editors do more than your sentence suggests. They also (sometimes) edit peps, (sometimes) provide advice about preparing one, and commit and update peps when the author(s) do not have commit access.
The distinction in the types of email that PEP 1 is attempting to make is the distinction between public discussions of the PEP (which go to python-dev or python-ideas) and semi-private discussions about the mechanics of preparing and posting and updating PEPs, which should go to all the pep editors (thus the instruction to cc the list).
In other words, if you want non-editors involved in the discussion, post to python-dev or whatnot, and do *not* CC the PEP editors. All other PEP matters should go to the PEP editors, and *not* be CCed to any other list.
That's what the original text says, but admittedly you have to parse it like a computer would to understand that :)