My sense is that the part about credit/collaboration was actually somewhat overblown among the detractors. What roughly happened as far as I can remember is that JT and another person worked on mmap together with about equal contribution, though the other person might have been the one to have initiated the idea (and solicited help to push it through); then at some point JT decided to make a PR to the main repository in their own name, but crediting the other collaborator as a coauthor, which may or may not have been coordinated with the other person. After that, though, in a fairly characteristic fashion, JT started fielding adulatory questions from their fans (on Github, but also on HN, Twitter and possibly other media) about the change, and quickly switched to simply referring to it as their own, with no mention of the other contributor. The other contributor expressed some misgivings about having their contribution erased, which were picked up by a growing set of people who were generally resentful about JT's conduct in the project. As far as I can tell, when confronted about it, JT at no point explicitly denied what the other person did (and I think the commit logs should all still be there in the fork), but at some point the other person just decided to stop pushing the issue due to being uncomfortable with becoming a playing ball in the fandom war between JT fans and antis.

My personal main gripe with JT really was the tone they adopted in the Github discussions, and the effect of the large numbers of drive-by supporters, who were often far less restrained in both unfounded claims about Justine's accomplishments and attacks on any critics. (At this point I'd also like to note that I consider some sibling comments to be uncomfortably hostile in a personal way, like the "hit piece" one.) I think that as a public persona, especially one who actively pursues publicity, you have some responsibility to restrain your followers - Justine, I get the sense, instead uses them as deniable proxies, as also seen with the instances where instead of straight up putting their signature on the "RAM usage reduced to 6GB" claim they instead choose to post a collage of screenshots of supporters making it.