Some of the best/easiest ways for someone with experience to find vulnerabilities is to throw automatic analysis tools at it. Many of which do work on binaries/disassembly, too.
Don't forget that in many cases it's not too hard to disassemble binaries.
Sure having the source can make it easier, but then things being open source also means that non evil actors might look at you code and help you find/fix problems before they become big. Which I believe somewhat balances it out. (for larger/widely used projects).
Like imagine log4j becoming known because of multiple supper wide spread attacks happening all over the internet, instead of getting known and then fixed and then mass attacks happened because updates take so long. And the log4j vulnerability is totally discoverable without source code. Idk. who found it but I wouldn't be surprised if someone ran into a non-security bug triggered by it and was, hey, wait a moment. Even if it didn't happen like that, it totally could have(1). Closed source would not have helped at all. At best it would delayed it and then made the effects worse due to coordinated wide spread attacks before there is a fix/mitigations.