By this definition, linux is a fork of windows and is inferior because it cannot be merged back to windows.

Often, starting from scratch is better. This is especially true when the goals or philosophy of the two projects are fundamentally different and incompatible, even if they perform similar tasks. Again, linux vs windows example applies.

If it can't be merged, it's not a fork, that's the key part of forks (well, not entirely, but the lack of shared code means it's not a fork by my definition).

That said, you're on point: this is forking the community. A hard fork, too.

That's great, except forks can be merged later.