The article says they aren’t sure about the details of the etching process, so I had a look at the patent which says briefly, in a paragraph towards the bottom

The article says they aren’t sure about the details of the etching process, so I had a look at the patent which says briefly, in a paragraph towards the bottom of page 16 / column 8, that they use a bath of hot phosphoric acid. Phosphoric acid is one of the main flavours in cola drinks.