If someone wants to find a vulnerability they will find a vulnerability.
only if there is a vulnerability. Which is possible but not necessary.
The only advantage open source maybe has that it’s harder to hide vulnerabilities for years
Vulnerabilitues should never be hidden but in stead eradicated. The true advantage of oss (regarding security) is that your implementations have to be secure. Security by obscurity is simply not an option
and it’s more obvious if they don’t fix it.
Security also means knowing about issues. When “they don’t fix it” in OSS, you at least (can) know about that, in closed source it is harder to be sure that the code is secure.