The key is illustrated in the book club parable: The elitism is directed outside of the group and becomes only a means of alleviating the fear of judgement for misjudging the paper. The grad student's approach clearly communicates the socially agreed upon reality: the whole paper is crap. This stance and boundary provides a clear decision space to the learning junior members: "if you think you see a mistake, those here will be happy to hear it; no sacred cows".
Bringing this practice into a situation where the target is a member of the group's work changes the dynamics such that you have to mind your Ps and Qs again -- and so, dampens learning.