By the time the XML specification was standardized from the more general SGML, I don't think there was any idea of using it only as a markup language. The XML specification lists support for a wide variety of applications as one of the design goals. Even schema validation, which long preceded XML, is something more commonly used for data not text. By this point XML is so rarely used as a markup language (by comparison with its usage as a data format) that the "Markup Language" part of the title is almost a misnomer.