As many studies have done, this one measured instructor intervention in only a quantitative way. As I understand, in order to "remain objective" the contributions where kept standardised and delivered at a particular time in the week. As such they did not form part of the flow of the interaction, and did not address students individually. Qualitative study may identify ways that acknowledging individual input while encouraging contributors to ellaborate or expand their ideas may contribute to a growth-orientation. Complementing specific behaviours may increase the likelihood of those behaviours in future. Perhaps brief discussion or a survey after the study would have helped researchers understand the students' experience better.
Reducing input to quantitative measures loses much of the data. As the authors themselves describe, the group may have posted fewer comments, but those comments may have been of a higher quality in some way not measured by the 3-point scale used. The researchers took some time to follow guidelines of previous authors, for example to give the group time to build relationships, but do not comment of the nature of the relationships developed or demonstrated during discussion.
Better relationships and feeling "safe" enough to give one's opinion may be more important than instructor's input. It should not be assumed that, being familiar with social media etc,, people feel safe online. I have read studies where participants have withdrawn after the group disagreed with a statement they had made. People should be assisted to be resilient and see the sharing and critical analysis of ideas in a safe light. Even when someone else makes fun of one's name, it is possibile to continue ahead.