In addition to Videonauth's answer, which points out that the site mentioned is popular with Ubuntu users and linking to it is not normally considered spamming, I would suggest that we can't actually tell whether the author of the post is affiliated with the site they link to. The condition you mention in your question title:
Should a post be flagged as spam if an author is clearly *not affiliated* with the promotion?
is really difficult to meet. How do we know someone is not affiliated with a product or service they are promoting, unless they say so, and we decide to believe them? If the author of the post in question stated
Disclaimer: I am the author of this OMG Ubuntu article
Then the post would be not spam by definition (but it would still be considered NAA by most reviewers IMHO). This is a bit worrying, because it suggests that if spam of the most obvious kind were to start appearing with disclaimers like "I am being paid to promote this product on random websites", we would not be able to flag it as spam... I guess we'll cross that bridge when we come to it.
Anyway, in deciding whether something is spam, I think affiliation of the author only matters insofar as it is suggested by something in the post, but is not stated. There may be nothing in the post that suggests affiliation, but it may still be considered spam.
Sometimes a post "promoting" something without disclosure of affiliation appears to be a good faith attempt to contribute to the site. Judging whether this is the case probably involves considering what is being "promoted" and evaluating whether the post tries to answer the question. Extolling the virtues of your favourite open source application would probably not usually be considered spam, but a post linking to a Windows-only non-free product site probably would be...
In this case, it's the content of the post and the link, rather than anything relating to affiliation, that makes it seem made in good faith rather than an abuse of the site.