Why does the Microsoft Office Custom UI XSD schema not compile using xmllint?

Question

I've downloaded the XSD for version 2 of the Office CustomUI specification from this Microsoft article.

If I use macOS xmllint to validate the simple text XML doc below, using the above schema, I get a bunch of "Duplicate attribute" failures as shown in the screenshot.

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui">
    <ribbon>
        <tabs>
             <tab id="TabTest">
                <group id="GroupTest" label="TEST">
                    <button id="ButtonTest" label="TEST"/>
                </group>
            </tab>   
        </tabs>
    </ribbon>
</customUI>

Microsoft's own XML 6.0 library parses the XML document against the XSD successfully.

Why doesn't it work on macOS with this command syntax from libxml?

xmllint --noout --schema customUI.xsd customUI.xml