On the face of it, my impression is that excommunication, if it happens, is more likely to be caused by the author being burdensome to deal with, than code forking per se.
Of course the author is unlikely to present it that way.
And although I felt the author was moralising in this critique, it may be they are not particularly rude the rest of the time. But even writing a plethora of well-reasoned but difficult to handle posts about what's wrong with your project can be too much.
Well-established languages, platforms and projects such as Linux that have a large labour pool with socially-established patterns of working don't have the same problem, because there's enough labour to deal with it.
Smaller projects just have to turn people away when they become hard work beyond the capacity of the project's core maintainers to handle it, though.
The solution found via FLOSS is to license software so that forking is always possible when a project cannot sustain different visions for the project's direction. This diffuses the tension when people have incompatible needs from the project. Sometimes it comes with drama, particularly if people are competing for attention and trying to persuade others to follow them, but that seems inevitable because of the competition.
It is still understood that forking is permitted and intended to be part of the solution, and the social niceties are that you may be encouraged, perhaps strongly, to go away and run your own fork yourself with your own resources, under a new name/domain/etc. while acknowleging where it came from. Then it's your own job to build a reputation too; it's only fair.
It's very difficult to keep running a project while your competition lingers on the same mailing list, constantly funnelling people towards their fork in the hope of making it more popular. That's a very good reason to "excommunicate" some people, or to forbid some topics such as advertising the other project repeatedly.