The same argument you made could be used if the author used the unicode codes for Ctrl+D, which I find to be an indication that the argument was a bit weak. The article itself is written in English, and to expand upon your metaphor, tt would be strange to use the term agua to refer to water in an English article. Language is really based around what people use, so using what people know is generally considered good communication. If you use something different it's considered good form to explain why. Each representation you chose is effectively shorthand for the English phrase 'Control + D', just with different stylistic representations. People are much more familiar with Ctrl+D to refer to the key combination of Control and D.
Anyways, I had the same issue when I first started using Linux. It probably took me over a year to realize the caret stood for ctrl, and that's only because I accidentally typed it in something that printed the codes literally.