remove the words that don't add to the meaning of the sentence. that aside, this is in an academic setting and not prose where the rules are not hard and fast.
Several of the changes both in that article (“The reason that General Lee invaded…”) and in other examples from this topic take out relevant information from the reader about emphasis of what’s being said.
Perhaps (theory just concocted!) taking these helpers out puts more effort on the reader, thus putting the text on a higher relationship of authority with the reader, and that makes it seem “stronger” writing!
I can get behind this. Interesting that all three examples provided are improved by removing a "to be" verb. Reminds me of: https://en.wikipedia.org/wiki/E-Prime
Perhaps (theory just concocted!) taking these helpers out puts more effort on the reader, thus putting the text on a higher relationship of authority with the reader, and that makes it seem “stronger” writing!