> WSDL/XSD specs in this case seem to be incomplete or just wrong.

Are you meaning in my comment, or in the GitHub repo the article references?

Asking because AFAIK the theory of having (accurate) vendor provided WSDL/XSD files is that you can generate a bunch of code (ie parsers, strong validation) that works with it, generate tests. There are additionally well developed SOAP specific GUIs (ie www.soapui.org) which will directly load the WSDL/XSD files and let you immediately make test network calls to validate things.

Though sure, you can do the above manually without WSDL/XSD files, but if you have accurate ones provided to you by vendors then it makes things a lot easier.