No I don’t think you said I was entirely wrong, that part was clear enough.
My issue is more with your argument from authority and personal experience. It is very easy to be biased by personal experience, especially when it brings good money.
access controls and supply chain management and traditional security mechanisms.
So I’ll put my personal experience too (which is also a low value argument). From the outside it may seem this is well done in big companies. But the reality is that this is often a big mess and security often depends on some guy, if any, actually having some standards and enforcing them, until they leave because the company doesn’t value those tasks. But since it’s closed source, nobody knows about it. With open source, there’s more chance more people will look at this system and find issues.
I don’t doubt some ultra sensitive systems like nuclear weapons have a functional closed source security process because the government understands the risk well enough. But I think there are way more closed source systems, at lower danger level but which still impacts people’s security, that are managed with a much lower standard than if they were open-sourced.