> 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.
I don't think that's true in practice. For proof, just watch any academic debate, ever.
The parent comment is stating a premise as a conclusion. Not having trouble admitting when you don't know something or understand something very well is actually a third attribute (commonly called humility), not the result of the first two. If more people focused on having it as an attribute then the scenario that the parent comment mentioned (and similar scenarios -- such as disappointing YouTube comments?) would be much rarer in occurrence.
In fact, failing to have the third attribute affects the other two. Your confidence will quickly become overconfidence or pure hubris when faced with something you don't know much about. Also, in relationship to the second attribute, it is hard to convince yourself that you are not actually knowledgeable of something if you are not humble enough to consider that possibility.
I don't think that's true in practice. For proof, just watch any academic debate, ever.