Seems to me he is abusing the Accept-header. In his example, the application provides a JSON-response, but with a vendor-specific media type. Once the API version changes, he changes the media type again.

This is both impractical because if the client application consuming the content happens to be a web browser, it will not see JSON and at least to me seems like a major flaw: if it is JSON, use the appropriate media type.

If you need to fiddle with HTTP headers, probably a X-API-VERSION header would be more logical instead of clobbering the media type.