Given the Linux initramfs targets a block device as a file that then gets mounted as the persistent root filesystem, I don’t think it would really be possible to unmount / and replace the location with a file. Root isn’t represented as a file or directory in any filesystem structure and is a construct of many Unix and Unix-like kernels.

Certainly a failure but at least it wouldn’t actually be as harmful as it reads, given / is a directory and the assumption you’re not root.