On the neutral category: the model outputs continuous scores from 0 to 1, so neutrality does exist around 0.5. The bimodal distribution with peaks at roughly 0.0 and 0.95 reflects how HN users tend toward strong evaluative positions. Three-class models could provide additional perspective, and that's worth exploring in future work.
Also love your meta-observation. Imo your comment is critical, substantive, and engaging. By sentiment metrics it's "negative," but functionally it's high-quality discourse. But that's exactly how I read the data: HN's negativity is constructive critique that drives engagement, not hostility.