This is correct, but you are missing the point Lord_Evil wants to make. GitHub allows the dash, so the GidHub field in the forum should allow it, too.
Here is the rule for GitHub user names:
Username may only contain alphanumeric characters or dashes and cannot begin with a dash
(pasted from GitHub)
Under this rule, “Lord-Evil” is a valid GitHub user name.
This means, Lord_Evil made a valid bug report and the forum rule for the GitHub field must be changed.