Sometimes fixing upstream is hard / not possible, when maintainers don't want to accept others' proposals/vision, or are cautious to change architecture with breaking changes.
I don't know much about this particular case. However generally speaking that is not true. They would just need to make the fork public. Whether the maintainer accepted the changes upstream is irrelevant.
Unless they actually tried to upstream it, this point doesn't really matter. From what I can see, they never actually tried or intended to upstream their changes, so this is irrelevant. They didn't even release the fork's source, let alone try to upstream it.