I apologize if the design description was not clear about HTTP status codes. Yes, of course HTTP status codes will be returned (since REST is nothing more than HTTP 1.1).
What I tried to describe is the HTTP status code behavior when an OFBiz service returns an error: The service error will not affect the HTTP status code. Instead, a 200 HTTP status will be returned and the response body will contain the result of the service call - which describes the error returned by the service.
The reason why OFBiz service errors are handled this way should be obvious from a web client perspective. If an OFBiz service returned an error and that error resulted in an HTTP error status being returned, then how would a web client (or developer) know what the service error was?