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Jonathan Sampson was already a moderator when he posted that question, and moderators were exempt from being required to tag a question with one of the required tags. It was still recommended that they did, though, since any normal user who edited the post subsequently would have had to add one.
Perhaps because of that situation, moderators are no longer allowed to leave off required tags as Bill the Lizard points out.
Tim has the right answer for this case, but this can also happen when a question is migrated in from another site.