I'm sure it's up for interpretation, but it's a reasonably defensible position. Here's what the XML 1.0 Spec has to say:
Abstract
The Extensible Markup Language (XML) is a subset of SGML
that is completely described in this document. Its goal is
to enable generic SGML to be served, received, and
processed on the Web in the way that is now possible with
HTML. XML has been designed for ease of implementation and
for interoperability with both SGML and HTML.
...
1. Introduction
Extensible Markup Language, abbreviated XML, describes a
class of data objects called XML documents and partially
describes the behavior of computer programs which process
them. XML is an application profile or restricted form of
SGML, the Standard Generalized Markup Language [ISO 8879].
By construction, XML documents are conforming SGML documents.
XML documents are made up of storage units called entities,
which contain either parsed or unparsed data. Parsed data
is made up of characters, some of which form character
data, and some of which form markup. Markup encodes a
description of the document's storage layout and logical
structure. XML provides a mechanism to impose constraints
on the storage layout and logical structure.
...
1.1 Origin and Goals
...
The design goals for XML are:
XML shall be straightforwardly usable over the Internet.
XML shall support a wide variety of applications.
XML shall be compatible with SGML.
It shall be easy to write programs which process XML documents.
The number of optional features in XML is to be kept to the absolute minimum, ideally zero.
XML documents should be human-legible and reasonably clear.
The XML design should be prepared quickly.
The design of XML shall be formal and concise.
XML documents shall be easy to create.
Terseness in XML markup is of minimal importance.
It seems to me that the real problem with XML is CDATA sections which have been used to cover a multitude of sins. CDATA should have been implemented as (say) normal XML with a reference (offset and length) to binary data at the end of the XML document. No special escape sequences would have been necessary, and the XML body could be kept clean.
The single and only interpretation that goes from that to "designed for editing text documents" is the fact that the authors called them "XML Documents". But XML Documents != "text documents". XML Documents are big property bags of data, and have no correlation with "editing text documents". The primary use of XML was, and remains, data exchange, where system A generates data that is consumed by system B.
Abstract
1. Introduction 1.1 Origin and Goals http://www.w3.org/TR/1998/REC-xml-19980210