I learned a long time ago that if you share your knowledge freely with your co-workers that only increases your job security. I postulate this plays off imposter syndrome. Your co-workers think to themselves, if this person is willing to share that knowledge, what are they holding back?

I agree but I'd place more of it on the fact that people like people who help them. I've worked with a few engineers who have been incredibly helpful and gone out there way to share knowledge with me. I consider every single one of them to be a great person. If anyone ever mentioned them in a conversation I'd like say 'oh they're great, I love working with them' or some variation of. When it comes to office politics, like it or not, it exists and likeability is important.

I agree and have wholeheartedly embraced this idea as a manager.

At the same time, I once had a manager who dinged me in an annual evaluation for asking too many questions and not knowing as much as I should about our applications as a senior developer.

When I pointed out that I had written most of the documentation for those applications in our wiki, he replied, "That's not knowledge. Knowledge is what is in your head."

The obvious lesson: avoid organizations like that.

I learned the same, and follow such. I've also seen orgs where there is no version control because the culture encouraged people to consider themselves as sole sources of indispensable knowledge, irreplaceable. Those tended to be toxic places, but as such they show a counterexample where their specific job security was from not sharing their knowledge and experience.

I don't think this has much to do with imposter syndrome. A person who can scale their knowledge across an organization is simply a more valuable employee than an equally-skilled person whose knowledge is mostly not shared.