that was my source, but anyway I will copy-paste the simplified explanation (the one I understood, btw)
I might be oversimplifying, but here goes.
The Board Community Council reached a decision on something (namely, giving members of the Community Council further direct GitLab access); Hans happens to be the one currently endowed with Owner privileges on GitLab aside from Ciaran; Hans refused to implement the decision, while arguing that the decision hadn’t been clearly taken, that the meeting minutes didn’t specify it, or such technicalities. The Community Council clearly disagreed with that view, and, seeing that they are in theory given the ability to make such decisions, but that in practice, Hans has the technical privileges to block, override, and trump them, they considered it a farce, and resigned from the Community Council, as well as leaving the F-Droid project in its entirety (except for Izzy, who however also resigned and signed the statement I link in the OP, and who is taking an indefinite break).
I guess they didn’t realize, and didn’t like, that their project was being run by a self-appointed leader rather than by the structures that had just been carefully put in place to avoid having unclear leadership.