But they have your upvotes, saved links, private messages, and at least some idea of which outbound links you've clicked (you can turn some of that off).
It's like an ISP knowing a significant portion of your web browsing behaviour.
Of course they have that data - you have chosen to send it to their website. It would be physically impossible for that website to run if the data could not be read. Obviously, it's preferable if only 'the right people' can read it, but it's much safer to assume that isn't the case.
If you're concerned about the outbound link issue, disable javascript. A cursory look at the page source should convince you that that's the only way they can track outbound clicks.
The parent commenter was saying anybody can go through your comment history, and by extension admins have no special access to 'dark secrets'. I was pointing out that that's just not true.
It's like an ISP knowing a significant portion of your web browsing behaviour.