The Origin of X-to-Close in UI Design

The author missed one: Wordstar

See pg. 39 of this .pdf file http://bitsavers.trailing-edge.com/pdf/microPro/Wordstar_3.3...

"X" is used as the "exit wordstar" command menu option.

This was WordStar 3.3, in 1983. WordStar existed for far earlier than 1983, and likely always used "X" as the "exit wordstar" command.