While you can argue that some piece of open source software can be more insecure than a proprietary alternative, auditing a piece of software requires access to the source code and that is mandatory. And with open source everybody can audit with no restrictions. Yes, OpenSSH is a piece of shit, but how do you think it was discovered, from 2 independent parties no less.

Then there's another effect that I like - after the initial patch was released, the story went public, we got notified immediately, then we could discuss about what caused it and see the actual commits and who did it. Such a catastrophe can sink a company, therefore you never see such post mortems for proprietary stuff. And yes, even I as a developer cannot audit software for security, but the point is that I could hire somebody else to do that for me, like the Finnish company that discovered Heartbleed.

So yeah, there is no concrete proof that proprietary stuff is less or more secure than open source, but the point is that we'll never know, because nobody can know how secure something is without looking at the source code.