The authors are assuming that they are allowed to use custom fonts. So if they need eight different variants of any particular glyph, they can just create eigh

The authors are assuming that they are allowed to use custom fonts. So if they need eight different variants of any particular glyph, they can just create eight different custom fonts where each glyph has the desired appearance. And yes, if you received a Word document with custom fonts, you might be suspicious of hidden information, but the authors are further assuming that the document is only distributed in a format where the use of custom fonts is not readily apparent. This is why they talk about extracting the information from "an encoded document stored as a vector graphic or pixel image, or even on a printed paper".