This advice, as with much other good advice, should be taken and understood in context. It is an essay about how to write a good essay, and there are times when you want to break some of these rules.
As one example, expressions like "I think that" and "in my opinion" can be useful if used purposefully. When reading an argument, we have a tendency to fixate on the points that we take issue with personally. By saying "I think X is true" instead of just "X is true", you make it easier for someone to disagree with your specific point rather than your entire argument. In some contexts (like HN comments), this can help to reduce animosity and get much more of your point across to people who don't fully agree with you.
As one example, expressions like "I think that" and "in my opinion" can be useful if used purposefully. When reading an argument, we have a tendency to fixate on the points that we take issue with personally. By saying "I think X is true" instead of just "X is true", you make it easier for someone to disagree with your specific point rather than your entire argument. In some contexts (like HN comments), this can help to reduce animosity and get much more of your point across to people who don't fully agree with you.