Hugh, the source isn't public, but I can say that the Toolkit doesn't alter the data returned from Data ONTAP. The API XML is parsed directly into the objects that the Toolkit emits. Here's how you can exercise the API directly to inspect the raw data:
Are those commands, like "<volume-list-info/>", taken from the Manageability SDK?
Yes and no. Those "commands" are Data ONTAP APIs, which are defined independent of the SDK. Having the SDK is not required to exercise the Data ONTAP API interface (the PS Toolkit doesn't use the SDK). But the SDK is where you can get the API documentation.