

If open-source, a lot more eyes could be on it
On the source code. Absolutely the same amount of eyes on the binary.
Anyway, there’s a joke (by Linus Torvalds, I think, but maybe I am wrong) that most of the eyes that could look at the code are attached to hands typing the thing about “more eyes”.
and therefore the chances of intentionally implemented vulnerabilities
Source code being available is obviously beneficial for learning how a program works as a whole, or participating in its development, obviously, but for finding things hidden I’m not sure.
Once again you are talking about programmers in general and not security researchers.
I have had a look. I’ve also done some solving of simple crackmes and such. I’m definitely not competent, but to find a security backdoor well-hidden you’ll have to examine behavior, which requires certain skills, and then you’ll have to look at the executable code, and then, of course, having the source is good, but less so if it’s deliberately made look like normal.
I think I’m mistaken on that attribution, OpenBSD’s Theo de Raadt is more likely to be the author.
Yes, I agree that it’s better when the source is present. But if you overvalue the effect, then it might be worse. Say, again, with Linux - plenty of people are using thousands of pieces of FOSS software, trusting that resulting thing far more than Windows. If we knew that the level of trust is absolutely the same, then one could say Linux is safer. But we know that people sometimes do with Linux all kinds of things they wouldn’t do with Windows, because they overvalue the effect of it being FOSS. It’s FOSS, but you still better not store 10 years of home video unencrypted on the laptop you are carrying around, things like that.