I have the same experience as you. I have probably worked with a few dozen SOAP services in my time, so perhaps I have been "lucky", but WSDLs have not been an issue for me.
WSDL is in some ways more user-friendly than openapi schemas. They have a rich set of types that mostly just work as expected. When I try to generate models from an OpenAPI JSON file, I often find myself either overriding the models or making scripts/plugins to rewrite the json to fix types. An example of this is using proper decimals instead of doubles, which is crucial in my line of work.
From what I can tell it is a library for standardising return values that is independent on the transport medium. A HTTP service may return a particular http code or body, and gRPC would have its own representation.
I think this library maps the different representations into a single representation. The business logic, metrics, logging etc. will use the single representation instead of being tightly coupled to the transport medium
Example from the docs where the "PAYMENT_DECLINED" state is mapped from protocol specific responses:
