I think you are correct. As you might already know, new watch faces using the Watch Face Format won’t have any code (see Watch Face Format | Android Developers). WearOS 4 itself will run your watch face and even have a built-in customization editor. The following screenshots are from the current WearOS 4 preview running in an emulator:
As you can see there will be many customization options available for watch faces: complications, color, layouts/styles, “on/off”-options. The text you see in this screenshots are provided by the watch face (“Graphite”, “Modular II”, “Bold Time” and so on). That’s what this “Text ID” in Watch Face Studio is for.
WFS will support more and more of the Watch Face Format (I hope). The technical documentation for those customizations are here:
UserConfigurations
Edit: If you want to test your watch face on WearOS 4, you can follow this guide: https://developer.android.com/training/wearables/versions/4/setup
[edit: This link is broken, reformatted to no longer autolink]