I'd argue OSS isn't sufficient on its own and that I suspect moderation only plays a small role. I think it's primarily the separation of roles. For a complete outsider whose only interest is exploiting users publishing a sufficiently popular piece of software and also gaining the ability to add things to the debian repos is a huge barrier. You'd have to invest years of work to do both of those things and then hope that no one happened to notice anything before it was too late.

Of course the FLOSS aspect adds an additional hurdle that this popular piece of software will have to somehow avoid having much of a contributor community around it since that would greatly increase the risks of your malicious changeset being reviewed. I guess what happened with XZ was about the best case scenario that an attacker could realistically hope for.