I personally do feel that we have to agree that CU is a rival. Neighboring state, pretty strong hate on both sides, and a few (though not very many) important and memorable games.
Thing is, though, K-State in the late 90s and Missouri now also meet those criteria. The truth is, CU-NU was a compelling rivalry for a few years in the early 90s (maybe early 2000s) when both teams were national contenders, but now its hard to consider it much more than any other division rivalry.
That being said, I do agree with the article that we do need a real rival. The Big XII kind of killed the OU rivalry (though OU's ineptness in the 90s and our recent suckiness haven't helped). It would be great if we could get good again and have another solid team in the division to duke it out with regularly for the crown. The thing is, though, must Husker fans would be just as happy if that rival turned out to be Mizzou or K-state (Lincoln is much closer to Columbia than Boulder, which means we have to put up with more Tiger fans). CU, I would guess, will only really ever have eyes for Nebraska.