Information set isn't a description of XML documents, but a description of what you have that you can write to XML, or what you'd get when you parse XML.
This is the key part from the document you linked
The information set of an XML document is defined to be the one obtained by parsing it according to the rules of the specification whose version corresponds to that of the document.
This is also a great example of the complexity of the XML specifications. Most people do not fully understand them, which is a negative aspect for a tool.
As an aside, you can have an enforced order in XML, but you have to also use XSD so you can specify xsd:sequence, which adds complexity and precludes ordered arrays in arbitrary documents.
There are well adjusted people that choose their behavior based on empathy and there are well adjusted people that choose their behavior based on what is allowed/normal. There are way more people in the second category