Depending on when you call it (if power supply is plugged in or not) will affect which one it uses. Now in the kernel with Ubuntu 18.04+ it has support to set both battery and AC timeouts. You would need to either unbind dell-smbios-wmi or unload it to cause libsmbios to fall back to dcdbas.
However due to WMI command filtering it's not allowed to run. It also contains a newer libsmbios in the repositories that can set timesouts on AC and battery both properly. In Ubuntu 18.04 it contains a kernel with dell-smbios-wmi, dell-smbios-smm, and dcdbas.The libsmbios in the repositories for Ubuntu 16.04 doesn't properly support timeouts on AC. In Ubuntu 16.04 the kernel only contains dcdbas which both the kernel modules and libsmbios use.I didn't get a chance to comment on this more over the weekend but it's a bit of a more complicated scenario going on.
