I understand the reason for this point, and certainly do not advocate the opposite. But with the best will in the world constructive criticism is always hard to take. We do not want to get into a situation where valid and important points are discounted simply because they are perceived as unfriendly: the question is whether they are helpful. Mechamisms exist for dealing with behavioural issues already.
Topic on Talk:Technical Collaboration Guidance/Community decisions
I think it doesn't harm to mention it.
MediaWiki.org posts and Phabricator tasks are editable. If the only problem in a blocker reported is the tone, the reporter and others can simply improve it. Dismiss a blocker only for its tone when it has substance would be wrong, but responding to unfriendly reports as if nothing had happened isn't right either.
Different communities might have different expectations for friendliness. It is not uncommon to see upset users landing for the first time in MediaWiki.org or Phabricator with a complaint, receiving quickly a remark about their tone regardless of how right their complaint might be. Adding the remark in the recommendation helps setting an expectation that goes in line with other efforts to improve friendliness and respect in Wikimedia technical spaces.