My bad, I thought that rule was about the medium (i.e. the post itself). I didn't it also applied beyond that (e.g. commenting about Slack's UX on a post whose topic is a subset of Slack's UX).
IMHO, your original interpretation is correct. The rule is about the UX of the article itself. In a story about Slack UX with respect to accessibility ... general issues with Slack UX seem topical enough.
https://news.ycombinator.com/newsguidelines.html