Yes, it's possible to use this from with a smartphone. The README did say something specific, like, how to do it. In the app go to Settings -> Linked Devices -> add a new one. Decode the URL in the QR code, then add it to signal-cli like this:
> To be able to receive messages signal-cli uses a patched libsignal-service-java, because libsignal-service-java does not yet support registering for the websocket support nor provisioning as a slave device.
So maybe you can add a smartphone if you've already registered via the CLI but not the reverse? The wording is unclear to me.
That line is explaining why signal-cli doesn't use the real libsignal, but a modified copy of it. They've modified it to support shit like that. However, https://github.com/AsamK/signal-cli/issues/49 makes it sound like you may have other issues.
signal-cli -u USERNAME addDevice --uri "tsdevice:/…"
As outlined in the README under the "Linking other devices" section