SGML was designed for documents, and it can be written by hand (or by a machine). HTML (another descendant of SGML) is in fact written by hand regularly. When you're using SGML descendants for what they were meant for (documents) they're pretty good for this purpose. Writing documents — not configuration files, not serialized data, not code — by hand.
XML can still be used as a very powerful generic document markup language, that is more restricted (and thus easier to parse) than SGML. The problems started when people started using XML for other things, especially for configuration files, data interchange and even for programming language.
So I don't think GP is wrong. The authors of the original XML spec probably envisioned people writing this by hand. But XML is very bad for writing by hand the things that it eventually got used for.
XML can still be used as a very powerful generic document markup language, that is more restricted (and thus easier to parse) than SGML. The problems started when people started using XML for other things, especially for configuration files, data interchange and even for programming language.
So I don't think GP is wrong. The authors of the original XML spec probably envisioned people writing this by hand. But XML is very bad for writing by hand the things that it eventually got used for.