There are many ARM devices on the market at the moment, with even more appearing regularly, and it's not possible to provide support for them all in the main Slackware tree.
The Hummingboard, by Solidrun, is supported outside of the official Slackware tree by the Slackware community.
The Cubox-i is totally compatible with the Hummingboard and can install and run the same Slackware ARM software.
Unfortunately, the community project that supported the Hummingboard is no longer active.