This is because it creates a normal file with tempnam and then renames it, but renaming between different streams is not possible:

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Filesystem/Filesystem.php#L457

I discovered this when trying to use a virtual file system (https://github.com/mikey179/vfsStream) for tests, trying to pass it as the kernel root dir, but not being able because the kernel uses the ConfigCache, and this last the filesystem dumpFile:

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Config/ConfigCache.php#L98