That's typically a nonsequitor.
Actually, it isn't - XML is cross-platform (but particular XML
applications may not be).
XML-based applications are commonly only implemented on one platform,
and for any given XML document, it is entirely possible that the only
application that has been written to use that document (DTD/schema)
runs on a single platform.
Again, here is the problem you seem to be having. XML is **not** the
application, it is the data format and the data format most definately
**is** cross-platform, even if the application software is not.
For instance, any of Microsoft's XML support has solely been written
for their Windows platform.
Also not true. For example, Microsoft Word (especially in later
versions) can write XML formatted documents rather than the
proprietary "DOC" format and I can (and do) read those in Open Office
on both Windows and Linux systems.
If you have a "BizTalk" document, it's more than likely that the only
place it's useful is when interpreted by a Microsoft BizTalk server.
BizTalk, to be blunt, is dead. It was only ever a Microsoft
proprietary method that tried (unsuccessfully) to make parts of the
XML standard into "Redmond-only" versions, much as they did with Java,
browser Javascript and others.
This time, it failed and Microsoft have abandoned BizTalk.