Regardless, I would keep the outbox focused on guaranteeing the messages transactionality.

However, message brokers are specialized tools, with more functionality and performance than a DB.
Not to mention the decoupling they provide, cozz using the DB means the consumer would need to know about the producer DB...
Maybe I'm misinterpreting him and going further than what he suggests...?

I felt the same. He is adding the list of subscribers and the point at which each of them are in the stream. Both Kafka and rabbitmq, and even Redis, already do that. So he is transforming the outbox into a message broker, where then the message brokers would be unnecessary.