I almost got a bingo by checking off things I’ve muttered to myself.

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.

If your software makes your clients’ life easier and your internal operations cheaper/faster/whatever, it’s a competitive advantage. Why would you give it away? Corporate greed or healthy competition, I suppose, depending on your point of view.