He says : "The description of paragraphs, styles, workbooks, worksheets, rows, and cells don't have anything to do with whether you are writing code for Windows, Macintosh, Linux, or the iPhone. However, there are parts of the Open XML specification that the opponents of Open XML have said are platform dependant, such as the specification of embedded, linked objects, but this simply isn't true"
He gives the example of ThinkFree.com, which allows users to access Open XML documents via a web interface, or through a rich client interface. The rich client interface is supported on Windows, Linux, and Macintosh systems.
And the same document in Excel 2007 : He also shows that ThinkFree's rich web client runs using the Java Plug-in 1.6.0_05 which uses JRE version 1.6.0_05 Java HotSpot(TM) Client VM. ThinkFree's server is a Linux server running Apache: Open XML embodies good cross-platform design in number of ways :
Open XML is based on standards that have been implemented on many platforms: XML and Unicode.
Open XML is based on the widely deployed ZIP file format, as documented in the PKWARE specification. Open XML only uses the DEFLATE decompression method, which the first and most commonly implemented compression method for ZIP files.
Several countries wanted improved interoperability with existing W3C standards by eliminating dependencies on specific Web browsers, such as Mozilla Firefox, Microsoft Internet Explorer, or Apple Safari. In the BRM, it was proposed to have a mechanism where applications can customize content for browsers according to their support for different levels of W3C HTML, XHTML, and CSS content.
Btw, I didn't knew about ThinkFree, which has nicely implemented Open XML. ThinkFree is a Korean company that is making an on-line office platform that uses the Open XML file format. It allow users to edit documents online.
For more details please check Eric's blog post Open XML Sets the Standard in Cross-Platform Implementation