Extensible means (to me) that it can be extended beyond its original design scope by adding new mechanisms.
I claim that this is not the case. XML has pre-defined syntax (begin/end tags with attributes that can be set within a tag). As such you can define any tags you want, and add any attributes you want, but that's not extensibility, it's in the original design.
There's no way I can see to extend the format without rewriting all the existing XML parsers.