It is when they setup an open source project that has that appearance and is framed as being such a project.
It is entirely the maintainer's responsibility to establish the type of open source project it is. If it was just a toy hobby project and that's all it was meant to be then it should have been framed as such (such as by being in a personal repo for starters). This project promoted itself for production usage, requested feedback & patches on its project page, and had a github setup that gave an appearance of being, for lack of a better word, professional. That's entirely the fault of the maintainer. They set all that up. They established the expectations of the project.
None of that at all forgives the name calling & mud slinging they were subject to, of course. Two wrongs don't make a right. But the maintainer was still also "in the wrong" here. They needed to hand off the project much sooner than they did when they realized they were not at all prepared or ready to handle what they promoted the project as being.