I may have a stupid question but... While I really enjoy Theo's talks and writings, I wonder if the fact that the VCS is CVS ain't a security issue in itself? It's been really a long time I haven't used CVS but I remember that attempt to introduce backdoors in projects using Git as a (D)VCS have been caught (it was in the Linux kernel I think). IIRC some attempts were caught precisely because it's hard to fake SHA ha…
Git and CVS are both just tools. They each provide a server implementation, but it's uncommon to use either of these for write access in large projects. It's more common to wrap CVS or Git with a different frontend like HTTPS or SSH. My guess is that the OpenBSD guys use OpenSSH.
This team is fanatical about security and process. I am completely comfortable with them using whichever tools they want.