A link to the post where you found that comment would likely make it possible to determine, for sure, why the user's name is not linkified.

Most of the time when a username is *neither* a hyperlink *nor* of the form "user###..." (where the #'s are digits), it means:

- The question, answer, or comment was created on a different site on the Stack Exchange network.
- The question it's associated with was then *migrated* to Ask Ubuntu, to be a question on our site instead of the other Stack Exchange site. If it's an answer, what I mean is that the question it's answering was migrated. For comments (as in this case), what I mean is that the post it's a comment on got migrated.
- That user *either* doesn't have an account here on Ask Ubuntu, *or* they have an account but it is not associated with their account on the other Stack Exchange site (which, from the perspective of the system, is the same thing).

In short, most likely this was migrated from another SE site, where the user's account there isn't known by the system to belong to the same person as their account here (if they have one here at all).