A PubSub whisper and Twitch Chat whisper are two different things.
A PubSub whisper lets you send a message from your EBS or the broadcaster to a specific user of the extension, and then your extension can do something with that (like display it).
Chat whispers are a different system entirely. I don’t have a lot of experience with those, but I imagine you would need a chat client set up on your EBS and your extension would have to be request UserID access in order to send whispers to specific users.
Hope that helps