I guess I would have expected that to be somewhere more intuitive in the Release manual (which I looked over)
It is also in the release manual. http://www.bacula.org/rel-manual/What_is_Bacula.html
Right there, under "Bacula Components or Services": 'In other words, the Storage daemon is responsible for reading and writing your tapes (or other storage media, e.g. files)'
Bacula backs up to a volume, be that tape or disk. They are both the same to Bacula.... Again, that is in the documentation.
I hope the false statement in the article is soon corrected.