I just noticed that you can PM someone but without the option for him/her to reply, this is what probably happened. To clarify, I got a question about the Panopticlick thread, but couldn't reply.
To not allow responses the second option can be clicked (see below). And this was high likely the case.
Yes correct. I just thought it was a bit weird to ask a question, and then don't give an option to respond. Perhaps he made a mistake, who knows.