[noggin]

I assume LibreELEC is built on top of Arch Linux?

Not sure - it's very similar to OpenElec - they have just decided to go down different avenues and have split the project. It's described as a Just Enough Operating System. I'd always assumed it was a custom distro specifically for OE/LE rather than being based on others. It doesn't, AIUI, have a repo-type installation system for instance (no apt-get, yum, rpm etc.)