A "Web Service" is just a generic term for an electronic service that operates over HTTP.
SOAP and REST can both be considered as a web service. Whilst SOAP is a protocol, REST is just an architectural style that operates over HTTP.
The NPI registry API can also be considered a web service, but it's clearly not SOAP, and it doesn't really conform to a REST architecture style either.
In fact if you look at the demo page you realise that the API is actually just a simple HTML form submit handler where the form values are sent in the GET request, as you can see here it submits all of the form values even if we provide just one form value...
You don't need to use a REST client library to work with this API, here is a simple command line demo just using the standard %Net library - I had to create an empty SSL/TLS configuration in the management portal named npiregistry as this needs to work over SSL.
For Ensemble you could just use the standard HTTP Outbound Adapter.