(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.
I'm coming to the opinion that downvoting should not grey out posts, or that there should be some number of downvotes (greater than 1) required before it starts. It should be harder to suppress constructive, on-topic posts just because a bunch of people don't like the point.