I think if you're truly (1) confident about your knowledge or mastery of a particular topic and (2) actually are knowledgeable, you don't have trouble admitting things you don't know. Here's what I really hate though. When you are around multiple people who are the type to claim knowledge about things they truly have no idea about. For example in social situations or work situations and you have multiple of these typ…
Absolutely on point. I started my engineering career at 19 years of age because I was so deeply involved with my love for building computers (as in, from raw chips) that I could show the VP of Engineering at this one company that I was good hire. I ended-up working there for ten years. Everyone else in the department was at least ten years older than me. One of the first things that was driven into my head by the "elders" was:
Always admit what you don't know. It's the only way you will learn anything new.
To this day I always appreciate people who are relaxed and clear about what they don't know (or are not sure about). I know I can have conversations with them. These people don't tend to get defensive and are not insecure at all.
The other side of your statement is that, in some circles, if you assert what you do know you are instantly labelled "arrogant". If you categorically know something I have no problem if you make sure I understand that to be the case. That's just the way the cookie crumbles.