"Overconfidence and underconfidence both imply a non-optimal amount of confidence."

Not in the sense of logical implication. The terms refer to levels of confidence greater or lesser than they should be, with the criteria utilized determining what 'should' means in context. The utility of the level of confidence isn't necessarily linked to its accuracy.

Although accuracy is often highly useful, there are times when it's better to be inaccurate, or to be inaccurate in a particular way, or a particular direction.

"You seem to have a pattern of responding to posts with unsupported statements"

I can support my statements, and support my supports, and support my support supports, but I can't provide an infinite chain of supports. No one can. The most basic components of any discussion stand by themselves, and are validated or not by comparison with reality. Deal with it.

"that appear designed more to antagonize than to add useful information to the conversation"

They're crafted to encourage people to think and to facilitate that process to the degree to which that is possible. I can certainly see how people uninterested in thinking would find that unhelpful, even antagonizing. So?