No, if the paper is indeed relevant to the question at hand, this is an appropriate post and should not be flagged as spam.
According to the site rules, it is actually okay to mention your own products in your answers, as long as you disclose your affiliation. In the example you mention, the poster explicitly states that they are the author of the paper in question.
I think it is reasonable to be even more tolerant of people discussing their own papers than commercial products, since they typically do not have a direct financial stake in a paper.
If the reference given is blatantly irrelevant to the question, you could flag as spam. If it's somewhat related and apparently offered in good faith, but is incomplete or actually misses the point of the question, you could leave a comment pointing this out, and downvote if you wish to do so in order to signal the problem to others.
If you don't feel you have the expertise to accurately judge whether the reference is appropriate or not, leave it alone. Someone who does will eventually take a look.
As a separate issue, it would be desirable for the answer to give a summary of what can be found in the referenced paper, in addition to the reference/link itself; this is a general Stack Exchange principle. However, a main part of the rationale for this is to ensure that the answer retains some usefulness if the link goes down. If the reference is to a paper in the archival literature, this should not be a concern. So I would not worry about that issue so much; the answer as it stands is not as good as it could be, but it's okay.