That's completely wrong. XML is specified as a subset of SGML (it says so in the preamble even) by folks who where also involved in specifying SGML. Moreover, these same folks (the "Extended Review Board" at W3C) also amended SGML to align with the XML profile of SGML in ISO 8879 Annex K aka the "WebSGML adaptions".

Also: SGML = standard generalized markup language