I think his notes about performance miss the full picture. Obviously gzipped XML and JSON aren't going to cause network slowness, but I've been on big projects that use XML as a data interchange format and XML marshalling and unmarshalling with validation was a bottleneck in the system. We used JAXB and loading the hundreds and hundreds of schemas in JBoss was so expensive that they had to bump up the memory to 32 gb on our personal computers. In the end XML schema validation had to be turned off (removing the whole point of it). Also, this was Java which is compiled and obviously very fast. Can you imagine the same thing in PHP or Python? It would be impossible. I'd be curious if the author ever did a large XML project in a dynamic programming language. I seriously suspect not and these languages (PHP, Node, Python) are the most popular and productive languages in the world.

XML is good at some specific things, but as a general use data-interchange format it is dead as a doornail and there is no good reason to bring it back.