The important part here is that they don't boot as many binaries from rootfs, and that allows them to remount rootfs as read-only later on the boot process. You cannot do this within the normal environment; you need to use a specialized (external) init script and tools (bootmenu, sshd ...)

That's not the FIASCO image you downloaded from Nokia, since you used THE UTILITY to unpack it. So they forgot to rename the file extension when creating the output file, or probably, they don't even care as the code for that is device independent -- big deal.

What are you trying to do? Copying the rootfs alone in Fremantle is mostly useless, specially with the PR1.2, as $HOME and some system files will be on the eMMC.
And if you want to do this from the device, you really need to boot into a special customized environment, or else you risk getting a corrupted backup (yes, there's a SLIGHT chance only, but would you really trust something like that as a backup?).