The inverse is often true thanks to Linus’s Law.
The article you linked seems to suggest that Linus’s Law is a mere suggestion, at best.
No one is suggesting that open source is inherently less secure, just that the vulnerabilities are easier to find, and thus easier to get exploited. For a third party reviewer there’s a lot of incentive not to report bugs they would find in banking software.
I almost got a bingo by checking off things I’ve muttered to myself.