Yea you can't downvote submissions.

Now, the post linked itself has been upvoted so it doesn't have negative submissions but it seems to have had been the case.

I don't quite understand how posts can have negative points :-/

A hunch but maybe a post which got confused as comment id of some other comment which got downvoted so you had negative karma, but I am unsure of this spooky action at a distance. It would be really interesting to know the reason behind it.