It might be considered softening, but I think it's always essential to express your "confidence level" when giving feedback. The article does advise focusing on facts and tying feedback to outcomes, but managers don't have a godlike view. You don't necessarily know all the facts of the situation, and you can't always be certain of the business outcomes, especially when giving feedback to a highly-skilled team member who might have their own reasons for doing things a certain way.
Presenting feedback overconfidently can provoke defensiveness that humility would have avoided.
In a way, what the article is describing is sort of like building a scientific hypothesis: your feedback needs to be concrete enough to be falsifiable.
You don't have to know all the facts, and you don't have to come across as overconfident. You should present the facts as you understand them. This gives the other person the ability to contextualise the feedback you're trying to give them, but also the necessary context to contradict you if they think the feedback is wrong. Likewise, putting it in terms of business outcomes means that they can counter your argument with what their intended outcome was, where appropriate (which is an especially important conversation with senior/high skilled people).
All the caveats you're describing are pretty damn important, but they're all pretty much part of what the author means by "be kind".
Presenting feedback overconfidently can provoke defensiveness that humility would have avoided.