If anyone else hits this issue, which actually will manifest slightly differently these days with a message like this:
the-tool[375]: error: Failed to make path /dev/disk/by-partuuid/9696c9f0-4343-4d23-b345-b2e819c84b2 absolute: No such file or directory
[FAILED] Failed to start the-tool.service.
A more fool-proof way to diagnose what kernel module/driver is needed is to boot a working image (like the live ubuntu classic image mentioned here), and then follow instructions from https://unix.stackexchange.com/a/125272, essentially asking the kernel directly what module was loaded for the disk. Then file a bug against either initramfs-tools for Ubuntu Core 16 / Ubuntu Core 18, or ubuntu-core-initramfs for Ubuntu Core 20.