Whois lists Ceglowski Maciej as the tech contact, and he is the owner of Pinboard. It doesn't explain why he would let it point to a vanilla Apache install and not point it at something useful
I would say that leaving it pointing to a vanilla Apache install is pretty on-brand for Maciej. He's currently busy with election-related journalism, but I'm guessing that this thread will eventually get his attention and he will get around to having it redirect to pinboard.in.
"Election-related journalism" is a polite way to put it. And this has been the situation for what, 5 years? One of these days I need to take a weekend and get myself off that abandonware.