fastboot devices returned: (device serial.no) fastboot - I did however notice this returns like this even if the device was unplugged. I figured there had to be something wrong with the PC, so I rebooted it. After rebooting it seems all the commands work as they should!
fastboot flashing get_unlock_ability returns 1
fastboot flashing unlock worked after PC reboot, and fastboot flashing unlock_critical said it was already unlocked, so I could finally proceed to the installation of eOS itself.
I ran the install-script, like the page said chmod +x flash_FP6_factory.sh && ./flash_FP6_factory.sh
But it seems the script is stuck at sending 'uefisecapp_a' (192 KB)