The description of the spam flag is
Exists only to promote a product or service, does not disclose the author's affiliation.
In this case, the answer does appear to legitimately (note I'm not saying "correctly") address the question, so it doesn't exist only to promote a product or service.
There also doesn't seem to be any sign that the author of the answer is affiliated with the website.
Based on those factors, the answer doesn't seem like spam to me.
The only basis for even thinking it might be spam is the link. But this strikes me as a perfectly fine use of a link. The answer references some concepts that the author believes may be unfamiliar to readers, so it links to a website to provide further information on those concepts. Which the website does. That's exactly how links are supposed to be used. Now, the concepts in question may be totally wrong, sure, but that doesn't make the link (or the answer) spam. It just makes it eligible for a ton of downvotes, and that is exactly what has happened.