Ok, true or false: all novels written in the English language are great works of literature.
Obviously false, so why do some people believe that every schema written in XML is a great piece of technology? There are good uses of XML and there are bad uses of XML. For example, if you take a binary data format and just translate it into XML you will almost certainly get a bad application of XML.
Make sure you ask about how the schema was designed, where did the design come from, and how well that XML works with commonly used industry XML tools. XML has been around for a decade now. There is simply no excuse for bad design by professionals. Sure, there are some debates about what constitutes really good design, but bad is bad.
In the same way, just because XML is itself an open standard, that does not mean that everything created with XML is automatically an open standard. Anyone claiming that is either naive, misinformed, deceptive, or worse, in my opinion. Kelvin Lawrence gets into this over in his blog on IBM developerWorks. Take a look.