(https://news.ycombinator.com/item?id=13094354 is the comment, which I find entirely reasonable but obviously people disagreed, or at least it was considered off-topic)
There is no reason, based on the HN guidelines, that the referenced post should have been downvoted, let alone flagged. Whoever did so abused their power to make such decisions.
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[0] - Not sure what's the power of a vouch relative to a flag or a downvote, but my impression is that it's stronger.