I don't believe this is totally true.

If you wanted to have the sound go through the stereo, then yes you would need the Bose headunit, amps and speakers.

I've got the factory nav with an aftermarket system. The nav still works fine, just no voice (which I prefer).

 

Actually, all you need to do ids wire a small speaker to the audio ouput of the factory nav. Lots of people with aftermarket headunit and spakers have done this if you do a search.

 

A Bose unit is not a requirement. The track with nav use the stock HU.