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Well, that's different. You are talking about an "x" that you type in, as opposed to an "x" you can click on.

To answer your question, "x" (short for eXit) was available in vi from the beginning:

ZZ Exits the editor. (Same as :xCR)

Source: Bill Joy's "An Introduction to Display Editing with Vi" http://www.verticalsysadmin.com/vi/vi_editor__bill_joy.pdf

Yes, it's different, but the article was trying argue that since "X" wasn't used in that context at the time it was introduced as a GUI element, the GUI element couldn't possibly be referencing the letter as a way of closing a program. While I agree that it's unlikely that the GUI "X" refers to a letter "X", that's not a valid argument for that position.

Edit: and thanks for the vi ref.

Understood, and you're welcome!

Nano also uses Ctrl+X to exit.

nano is from 1999, so hardly relevant.

But nano is just a pico clone, and pico started out as pine's text editor. pine as a project was initiated in 1989.

ex uses x to exit as well (1976).