Doesn't follow at all. There are open source projects that expose read-only VCS and still only accept patches via email. Postgres, for example.
Postgres is a different project and they no doubt see a different cost/benefit ratio for public VCS, probably because they have many more core developers and contributors than Postfix.
I've actually had a patch accepted to Postfix and while I found it inconvenient not to use my usual VCS-based workflow, I also realized that the Postfix world revolves around Wietse, not me, and as a user of Postfix I benefit most when the project is optimized for him and not other people.
"The point is that public VCS doesn't benefit Wietse, and by extension Postfix, so why should he have one? A public VCS only benefits other people, most of whom aren't even contributing to Postfix development."
Like this:
"The point is that publishing source tarballs doesn't benefit Wietse, and by extension Postfix, so why should he? Publishing the source only benefits other people, most of whom aren't even contributing to Postfix development."
The answer to both question is "he doesn't have to, but if he takes seriously the goals of open source it would be silly not to." It's not unlike choosing to withhold your Makefiles, or documentation.