Obviously, it is unethical and most likely illegal to unilaterally, summarily claim copyright or relicense other people's contributions, and I appreciate the matter-of-fact tone taken by the issue opener and the other commenter.
However, I think it's obvious this isn't a scammer using open source as some kind of get-rich-quick scheme, putting in minimal effort into a fake open source project to attract highly valuable contributions in order to relicense them and make money off of other people's hard work. This someone who in all likelihood put blood, sweat, and tears into their project, gave it away for free on GitHub, accepted code contributions from people who in all likelihood were deriving a lot of value from Octotree (or else they wouldn't have had a pet issue they needed fixed in the first place), and has since grown frustrated by the time and effort costs, and the comparatively paltry returns.
Is their frustration causing them to lash out in a damaging and unproductive way? Certainly. Is it okay to renege on the implicit agreement with contributors that their contributions be under the terms of the AGPL license, and act as if those contributions were of negligible value? Certainly not. Is the maintainer right in their feeling that the value of those contributions is most likely less than the value given away for free to those contributors? I mean...probably.
I must really commend the noncombative, factual tone taken by the issue opener. And I hope the maintainer comes to their senses and deals with this problem the right way, by simply asking people nicely: https://news.ycombinator.com/item?id=24954772