I think it's entirely reasonable that someone using the same uncommon username on two services can be assumed as the same user, especially given this:
quoting for posterity. remember how i signedup with "Nimda" at coinchat and you thought there was relation to "nimda" here? just because the odds are low doesn't eliminate the doubt. the user kosmo kramer here? i know a guy on a local sports message board that goes by the same name. my point is people see cool usernames and copy them other places. it happens.