I didn't delete the comments on that question, so the mod who did would have to state their reasoning, but I would have also deleted that comment. Maybe not if I was just browsing the site, but if at least one comment on the post was flagged as chatty or no longer needed, I would typically review the whole chain of comments and clean up any comments that are not appropriate for comments.
I do agree with the concept of "never delete useful information", and I don't buy into the idea comments are ephemeral. However, comments should add questions or comments to the post that they are attached to. If it's not adding a question, a point of clarification, pointing out something to consider, or supporting information to the contents of the post that it's attached to, it's not a good comment and should be deleted so the relevant ones can be easily seen.
In this particular case, this isn't a good example of a comment. It does attempt to answer the questions and should be posted as an answer. If someone suggested that it should be a comment, they are wrong - that's not what comments are for. However, if it was posted as written as an answer, I would expect it to receive comments and/or down votes, since it doesn't do a good job of explaining why this is correct or a useful consideration.
Now, why would I do this? I think it's important to encourage the appropriate use of comments. Leaving something that resembles an answer as a comment doesn't encourage good behavior. It's why we quickly delete off-topic questions that have a very low chance of being edited to fit the scope of the site as well. Simply not seeing things helps discourage unwanted behaviors from others who may not know better.