The point I made was not really about monarchy, but about comment quality on HN. Low-information, high-indignation comments—such as repetition of well-worn political points—is the classic low-quality case that we're most hoping to avoid here. Especially because they tend to evoke even worse from others.
if you has said something like "we're not interested in this thread turning into another opportunity to litigate the pro's and cons of the monarchy" I would agree with you.
But specifically saying not to comment negatively while allowing positive comments on what is clearly a hotly contested issue is ridiculous.
https://news.ycombinator.com/newsguidelines.html