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).
###Why do I think unlinkified names usually indicate migrated posts rather than deleted accounts?
Every time I've seen a deleted account, its name was changed back to the original user###... name. I am not totally sure if this is always the case when an account is deleted, though it would make sense if it were; my understanding is that allowing users to delete their accounts is the way Stack Exchange respects our right under CC-BY-SA to choose to insist our names are removed from our work when published in a compilation.
Of course, most people most of the time prefer to exercise their right (also provided for by CC-BY-SA) to insist that their names remain on their work. For example, all of us right now are insisting that right be respected, just by not insisting our names be removed.
The exact circumstances under which it's okay for a user's name to be removed from the work are probably determined by CC-BY-SA and the Terms of Service, together. Perhaps user accounts that are deleted against the wishes of the users, keep the name. I'm not sure. I believe account removal other than by request of the user is quite rare, except in the case of spammers, whose contributions are also soft-deleted (thus visible only to 10k users, and generally not of great interest).