> Conversations happening with external assistants,
  > on the other hand, are interactions 
  > that Meta can’t analyze or monetize.

This would only be true if Business API allowed an external assistant to connect peer-to-peer to the customer, but I doubt that's case.

Otherwise, Meta can still both analyze and monetize any message going through the API, because it intercepts all messages coming through the API, and can edit them in place too (to inject ads, for instance).

So unless it is a P2P connection between external provider and WhatsApp client, this does not seem to be the reason.