The services are free to serialize the data in any way be it json, XML or even protocol buffers.
So Smithy is more comparable to OpenApi than Protocal buffers.
What's the use of such descriptions aside from diagrams? I realize there's e.g. terraform that use similar kind of language to describe what to create/destroy.
AWS sdks need to be implemented in dozens of languages so something like Smithy helps in code gen for multiple languages, avoiding the massively manual task of creating client apis.
Smithy is a much more abstract notation for defining services independent of any implementation details, and it is resource based rather than message based.