I’m talking about the setting available below:
Go into Settings > Companion app > Assist for Android > Enable wake word detection
I was expecting to see, after ‘Hey Jarvis’, it would also listen for the command such as ‘turn on the light A’ but all I see is chat box UI.
It’s okay because I might have been over-hyped for this and have not read any specific document. I just want to confirm if this is all it can do right now.
It is exactly doing that over here… 
Is your assistant configured correctly and able to process that command anyway, on another device or if you type the command in the chat box, does that work?
No, that’s not all it can do. It actually works as a voice assistant. I tested it and it performs my voice commands with no problem.
When you say “Hey Jarvis” and the UI box opens, it is in listening mode (or it should be) and your voice commands should work. At least, that’s how it works for me.
As @Nick4 mentioned, I would check that your assistant is configured correctly.