So I’ve fixed the issue. Turns out you need to specify a machine type when installing in order to pull a Hassio container that is specific for a Single Board Computer.
In my case, I elected raspberrypi2 as my machine type as my Cubox-i’s ARM Arch matches best that board.
Instead of reinstalling, I just deleted the generic “armhf” images and container and pulled the “raspberrypi2” ones. “io.hass.machine” label is now properly populated when inspecting Hassio docker container. I wanted to use raspberrypi3 as my machine type but I’m worried this subset of container might get updated to run on 64-bit native targets down the line, which the Cubox-i isn’t…
So to wrap it up, in order to have support for installing Add-Ons in Hass.io, you need to make sure that the Label “io.hass.machine” is present and has a value that is supported when inspecting the hassio-supervisor container. If there’s something wrong with that label, delete all containers and images related to hassio and pull some of a Single Board Computer that best match your current target. Adjust systemd units, hassio config in /etc and maybe stuff in your hassio config folder (/usr/share/hassio).