I think the point was that solely theoretical ideas could never have produced Mozart's Requiem.
However, there are plenty of other 'masterful' pieces throughout history that could not have been written through a mathematical or entirely theoretical approach, including some very recent music.
The way I see it, is that you don't have to be 'musically knowledgable' to be able to imagine incredibly complex music, but if you want to actually get it down on paper, then you do!
I know this is slightly off-topic now but check this out:
YouTube - Derek Paravicini - The Musical Genius Part 2
Derek is autistic and unable to comprehend musical theory.