That wording is just out of date, as it predates some of the changes to the API in general.
There are no pubsub topics that require no scopes.
All pubsub topics require a relevant scope
The full list of topics available to third parties can be found at PubSub | Twitch Developers
You are JSON encoding and sending a string? Not an Object?
This example might help, written in nodeJS https://github.com/BarryCarlyon/twitch_misc/tree/main/pubsub