This is my takeaway as well. Having the source code open makes it auditable, if not by you, maybe the community.
The free software license specifically gives the software an extra advantage in that changes to the software must be shared openly, if distributed as as binaries.
> source code open makes it auditable, if not by you, maybe the community
I think part of why this social engineering works so well is it takes advantage of that "many eyes" trust, where people are prone to delegating the responsibility of checking to the community and not do due diligence on themselves. I know I'm susceptible to it if I see a Github repo with more than 10k stars on it.