You can get the PDF yourself from CCIA's website or from Google's HTML version.

Here is the section that talks about interoperability issues and compares it to trying to follow a recipe:

INTEROPERABILITY LIMITATIONS WITH MICROSOFT'S XML FILE FORMATS

XML is useful for interoperability only if the schema is completely specified and is agreed upon by all parties. If the schema is incompletely specified or if parties cannot agree on the definition and format of the fields, interoperability will fail.

However, even if all parties agree upon the schema format, another interoperability consideration is whether or not all parties have the ability to create and consume the data types stored in the fields. If all parties cannot create or utilize the data types stored in all of the schema fields, interoperability will fail.

Recalling the XML schema example with chefs and recipes, suppose that a cake recipe calls for "one cup of marzipan." Both chefs may agree that the recipe contains "marzipan" in the ingredients list, and that one cup is required. However, if the American chef understands what marzipan is and the European chef does not, the European chef will get different results.

Further, if marzipan is an invention of the American chef, and the American chef refuses to tell the European chef how to make it, the European chef will get different results, even though he knows that the recipe is supposed to include one cup of marzipan.

Microsoft's disclosure of the WordprocessingML and SpreadsheetML schema is inadequate for interoperability because it omits critical information necessary for full interoperability. While these schema may or may not be complete, there is a fundamental limitation in that some of the items stored in the fields of the schema are Microsoft-proprietary data, and Microsoft has not disclosed the information necessary for others to interpret and use those data. As a result, we have a "marzipan problem" -- no one but Microsoft is able to create and consume the data in some parts of the schema, making the schema unusable for full interoperability.

Returning to our example, suppose the American chef has a recipe for the world's best chocolate cake, and makes it widely available, allowing and even encouraging other chefs to read and share the recipe. However, the recipe contains marzipan, and the American chef does not tell anyone how to make marzipan. As a result, even though you have the recipe, and even if you have the American chef's permission to use the recipe, you still may not be able to make the cake correctly.

And as to their conclusion, here you go:

While Microsoft has released a definition of the XML schema used by their Word 2003 and Excel 2003 applications, these disclosures clearly lack information which is necessary for interested parties to achieve complete interoperability with Microsoft Office 2003's entire feature set. Despite the fact that Microsoft promotes these disclosures as a prime example of their interest in supporting interoperability, the disclosures are incomplete and therefore effectively unusable; as a result, they have very little value as interoperability tools. Further, if these disclosures are being promoted as interoperability tools, but if in reality they cannot be used as such, one might wonder about the true motivations behind the disclosures, and indeed if those motivations have anything to do with interoperability at all.

I didn't say it, folks. They did. And what I understand them to say is that if interoperability is your goal, Microsoft's XML is not what you want.