HN also heavily penalises posts which have a high number of comments.

Right, but relative to the number of upvotes the submission does not have that many comments. And it fell just on the start of the workday (right?), see http://hnrankings.info/19031055/.

I am aware that's not a wanted discussion topic normally, but if behaviour related to an article about dirty behaviour smells dirty it feels a bit relevant.

It's best if you email us at hn@ycombinator.com, because we can look at the actual data. This is mentioned in the guidelines: https://news.ycombinator.com/newsguidelines.html.

Why is that?

To avoid controversial topics and flame wars (and likely to prevent heavy load on the servers since large threads cause significant strain on HN it seems)