If both success and error responses include the success field, then that can be a common discriminator between bodies of successful responses and bodies of error responses. Where this adds value beyond the request’s status code, I’m not sure. Maybe it’s useful in aggregated responses where partial successes are allowed (like POSTing a batch of objects)?
Your format looks half baked and not thought all he way the way through.
This does seem a bit heavily worded. There’s likely a reason they originally chose and continue to use that format, and it could be as simple as “all our other APIs use this format” or similar. There’s more to choosing a response schema than what is theoretically the most efficient way of communicating the information.