It has recently been revealed to me that the community policy discourages comments that explain down votes, at least on answers
Ehhh. NO. Not at all. I think you misunderstood. Because we agree that downvotes are a bit frustrating on a gamified site, we do encourage constructive feedback in comments. Things like 'Could you address the actual question asked', 'OP's goal is X, how and why does your answer help them reach this' or requests to clarify parts of answers are made daily and a lot here. To make sure these constructive comments get visibility and aren't snowed in underneath a bunch of discussion and chatter, we tend to delete every comment that isn't suggesting an improvement, requesting an author to clarify their post, or containing clarification that's not yet edited into the post.
We discourage comments calling out downvoters for the reasons EmC mentions in their answer there, and because the SE system itself is in favour of allowing anonymous feedback in the form of voting.
Calling people out doesn't really meet the criteria for 'allowing anonymous feedback'.
The discussion about forcing voters to leave a comment was had a few times on meta.stackexchange (the main meta for problems concerning all sites, not just IPS), the questions often being referred as a duplicate to Encouraging people to explain downvotes. The answer to that post also shows that there are systems in place to actually encourage commenting when downvoting, but that SE doesn't want to make this commenting mandatory.
Without some explanation of the down vote, the vote itself is meaningless.
Not really: The downvote button, when you hover your cursor over it, simply states 'this answer is not useful'. In this case, a blatantly off-topic question got an off-topic answer, in a sense you could say those 'aren't useful' on a site focusing on questions and answers about Interpersonal Skills.
Also, comments are used to suggest improvements. What kind of improvements could've been suggested here? 'Edit your answer to make it about Interpersonal Skills'? That's not really useful feedback on an answer to a question that itself isn't about Interpersonal Skills. It wouldn't have improved your answer in any way would you have made such an edit.
We're really doing the community a disservice with this policy. I would strongly recommend reversing it, unless anyone has some good reasons to the contrary?
As you seem to be a bit misinformed about 'this policy', I would say there's no need to reverse anything. I understand getting downvotes may feel frustrating, but comments calling out downvoters are not what comments are supposed to be used for. Instead, if you like feedback on your answers, you can visit the chatroom or ask on meta.
People here are generally doing a great job at pointing out improvements to answers in a constructive way, if there is a way to improve them. If they don't, as a moderator I regularly go through the recent answers and leave comments on the ones I encounter that were downvoted without explanation AND are salvageable, trying to help people improve their posts and setting a good example.