The directive to strike a positive tone could be nefarious or benign; we can’t tell from the info in the article. Every research paper has a tone that depends on the personality of the author - I’ve seen it even in my dry field of quantum optics. Every result has many implications and how you discuss them, which implications you emphasize, and the language you use will affect the tone, even if you are not explicitly furthering an agenda. A given article could have a range of legitimate tones, depending on who wrote it. Asking a researcher to be on the positive end of the legitimate spectrum is benign. It can of course go to far.