Even before Unicode, it was established practice that documents mixing Chinese and Japanese would use the same encoding for both and roughly nobody would bother to pick an ugly font for the foreign-language text to make it look appropriately different.
Unicode rightly decided that the fine details of appearance are left to fonts. Otherwise you'd also need e.g. a bunch of extra codepoints so that early-20th-century handwritten letters in German can have their look accurately preserved: https://en.wikipedia.org/wiki/S%C3%BCtterlin
Now, if a file was encoded in Unicode, and/or if it was in such document format that support inline font specification, such as HTML, then you could mix two languages without having to stick to one language by e.g. wrapping <font face=Helvetica>paragraphs and words</font> <font face=Futura>with tags</font>.
My point is, it seems that the author is not aware that each of CJK languages are only understood within each countries, in both writings and speeches, and that's somewhat peculiar.