I would also say that generally speaking you also get more eyes on your source code so you increase the likelihood that someone will find the flaw more quickly (although you could also say it's easier for bad actors to locate flaws to exploit too).
What I'd say is that given an equal amount of security effort an open source lib is more likely to have higher security, however by far and away the most important factor here is the amount of security effort employed and that is not generally correlated with the software being open source.
Also, a single company provides limitations - you've got blinders on, and your project isn't open for those with a different perspective to come in and take a look and notice something. I honestly think that fresh, open, and global perspective is truly key the success of OSS.
A large community of devs who are focused on security would indeed be good for a projects security, but that's not always their number one priority.
> your project isn't open for those with a different perspective to come in and take a look and notice something.
Yes, but consider the fact that a malicious party can also do this kind of analysis. For the record I'm not advocating for closed software, on the contrary, but merely pointing that the matter is more complex than it looks like on the surface.