I don't know a lot of serious systems security people who actually believe that. OpenBSD is fine, and smart people work on it, but it's been a long time since the early 2000's.
AFAIK it remains the single most selective platform out there. If a project is included in an OpenBSD release you know it has undergone serious whitebox scrutiny for security issues. I'm not aware of any platform that is quite so pedantic at the source code level.