Not to take anything away from Tim, who's contributions to XML were significant and quite possibly critical to XML's eventual success, but there is no sense in which Tim invented XML any more than any of us on the XML Working Group did.
If anyone can be said to have invented XML it would be Jon Bosak, who originally organized the Working Group and chaired it in a most masterful fashion, setting up working rules that allowed natural adversaries to work more or less harmoniously toward a common goal. If it hadn't been for Jon XML would never have happened and if it hadn't been for Jon's understanding of how to make this type of activity work politically, it might never have finished.
As an editor of the XML recommendation Tim's contributions were vitally important to the final shape and success of XML, but so were Michael Sperberg-McQueen's, Tim's co-editor. XML was very much a collaborative standard, reflecting the consensus of a wide range of constituencies. There were many of us at that time working on the challenge of a simpler, easier-to-process version of SGML for use with the emerging Web technology and it was Jon Bosak who brought us together and set us going in the right direction. Tim and Michael did a remarkable job of crafting the recommendation and getting it done and for that they deserve special recognition. But no single person invented XML.
An important aspect of XML is that in fact there is no invention in XML at all--XML is nothing more than SGML with lots of extraneous bits removed. XML is a tremendously successful marketing exercise, but it is not an act of invention, in that there is nothing in XML that was not already in SGML (all XML documents are conforming SGML documents by definition). The genius of XML is in what it left out, in adapting the big-iron SGML standard to the new 80/20 world of the Web that most of us on the XML Working Group were only just starting to fully appreciate. Tim, with his feet much more deeply in the Web world understood better than most of us what would and wouldn't be accepted in that environment and therefore had a lot of influence on what we chose to retain and that was very important to the success of XML.
Of course, many things have been invented around XML since the recommendation was published in 1997 and Tim continues to be a leader in many of those efforts and a thought leader in the Web world generally.
Founding member of the W3C XML Working Group