I'm not sure I'm fully getting this.
You say somebody downvoted your question and added a comment at the same time. If it's clear and obvious from the comment that they didn't understand the question and therefore downvoted, what's the problem about it? Obviously they didn't care to be identified as the downvoter.
Even more so: Downvoting a question or an answer actually kindly asks you to leave a comment. If you were able to identify the downvoter, this was by design.
Please correct me if I got something wrong.