Why this specific answer was treated as spam?

Question

I've recently flagged one post as a "low quality" post, well, because the "link only" answer option was not available in flag reasons. I'm talking about this post in which was a link to an external site, where was the answer to the asked question and so I don't understand why it was taken as a spam.

Could someone explain me, why that post was taken as a spam ?

^{P.S. I know the answer should be deleted but why just as a spam ?}

Answer 1

It was treated as spam because it is almost certainly spam. And as you said, it should have been deleted regardless due to being a link-only answer. Let's look at the evidence that this was spam:

1. It's a link-only answer.
2. This is the only answer from that user.
3. It was posted on an old question which already had a highly-upvoted accepted answer. There is no explanation of what was unsatisfactory about the existing answer.
4. It links to a blog post.
5. The blog post was posted a day before the answer, which makes it very likely that they have the same author rather than that someone happened to find the blog post in a Google search.

With all the evidence taken together, the chance that this was a post intended to drive traffic to someone's blog is very high.