Should I ask in the comments for the reason, or is Meta the better place? For bonus meta points, I’d like to know how to phrase an answer so that It doesn’t get downvoted.
You can ask for suggestions for improving your answer, but there's not much point asking specifically why it was downvoted. Voting is deliberately anonymous, so there's no way to make sure the downvoter sees your request, and no one else knows for sure why they voted.
If this is what you want, then you could leave a comment along the lines of "I'm interested to know how to improve this answer, and am completely open to constructive criticism." Most people aren't actually that tolerant of criticism, so a lot of readers are going to hesitate to comment for fear of getting into unpleasant situations, so you really do have to commit to being receptive if you want everyone to feel comfortable.
A couple things to keep in mind, though:
You already have a comment expressing skepticism about the core claim in your answer, as well as pointing out a clear factual inaccuracy. Someone may well have downvoted because of that, i.e. because they believe your answer is misleading or at least partially incorrect. Yet you're still asking about the downvote, so that could make it appear you might not be open to feedback.
In your question here, you say you would "like to know how to phrase an answer" to avoid downvotes. Downvotes are primarily for answers that are "unclear or not useful" (see the hover text). If an answer is unclear, sure, you can likely rephrase to avoid that. But if the content of the answer isn't useful (e.g. if it's incorrect or not a helpful way to address a question), your issue isn't phrasing. Assuming that phrasing is the issue can make it appear that you're not open to feedback about the content of the answer.
So in this particular case, I would suggest first focusing on verifying that your answer is in fact correct, and providing supporting evidence. Edit until everything in the answer is correct and well-supported. If something's wrong, remove it or correct it. If something's correct but people are skeptical, find a clear source to support it. Then if you receive further downvotes, I might proceed to asking for feedback.
I've added a comment to your answer with some specific potential starting points.
(Note that the original downvoter may or may not return, so even if you do everything perfectly, the existing downvote may remain.)