Looking into how to manage (in our case just list and get properties of) the SAML application that we have created via the "Web and Mobile Apps" section of the google console.
I don't see any reference to how to get this information from the https://developers.google.com/admin-sdk/directory/reference/rest documentation.
Is this API available somewhere else?
It is not possible. All you can use are the API methods listed in the documentation you shared, however you can only see the SAML apps from the Admin console web UI.
You can also request this as a feature using the "Missing features" option, to see if they have the chance to develop something like this.