That is not the main grub.cfg when traditional boot method, even when EFI. That is what I know and I did edit grub.cfg many times in the past years, not only Tumbleweed. If single OS (per computer, so no multi-boot), that file does not need to be there as a ‘smart enough’ EFI/boot/boot*.efi can find the grub.cfg on the rootfs.
But maybe have a look at (ignore that rEFInd, it is not openSUSE)