> So, we can't rely on checking ``self.done() and self._cancel_requested`` for future.cancelled() as this would mean that future.cancelled() would return true for a future that fully completed if `future.cancel()` was called after it finished (which is incorrect).

This should be "was called *as* it finished", not "was called *after* it was finished". If gather_future.cancel() is called after the future is `FINISHED`, it will immediately return false since it checks `self.done()` first (https://github.com/python/cpython/blob/b8f67c0923ac85468dfbfd43375e0bbfb6ca50ea/Lib/asyncio/tasks.py#L727).