This is an interesting question. There is nothing wrong with linking to another site, answering a question that already has a marked answer, or even answering a question that is old, but I think if you are going to do that it should be on something that either adds to the explanation or provides a different view point.
It is also useful to provide some sort of snippet from the blog post to show that the post is relevant to the question and allow the user to decide using that small amount of information to proceed to that site. In that particular example, the user essentially answers with just a simple link with no further explanation. The post itself isn't really off topic, but it provides no additional information to the post. It would have been much more helpful if the user would have posted something along the lines of:
I wrote a blog post about this. Here is the relevant information to this question:
Some extended quote from the post
For more information, check out my blog post here
Essentially, we need to make sure that the user adds good relevant answers to this site before they look purely for the page view on their own site. There is nothing wrong with having a blog and looking to generate traffic to it, but you need to provide more in your answer to be able to just link to your site. Unfortunately, without providing more details, it is essentially spam even though it is pointing to a relevant article.
EDIT: Just wanted to take a moment to record my results here. I downvoted that particular post, linking to this article as a method for the type of thing that should be added. The user read this post and revised their answer to make it much more SFSE friendly. Because of their efforts to remedy the problem, I changed my downvote to an upvote. This process seemed to work really well.