I'd say the position of Black's knight is somewhat iffy but not "just silly."  As far as I know, the standard view of that line is that it should lead to "+=," but the same can be said of 7...cd.

I'm not sure why 9. Bf4 was suggested as the best move, though it's considered one of the better ones.  (I seem to recall when 9. Bd2 was considered the best.)  I'm going to guess it's because it had the highest scoring percentage in a database based on 3 games, or some such.

There are two problems with 6...c5:

1. Black has to know how to play the Maroczy Bind after 7.0-0 cd4 8.Nd4 Nc6 and now both 9.Be3 Bd7 10.Nc2 and 9.Nc2 are slightly better for White in my opinion. As you pointed out, 7...Nc6 8.d5 Na5 9.Bf4 is insufficient for Black - the knight just looks silly on a5, a bit like in a Ruy Lopez Chigorin variation. 

2. After 7.d5, Black has to be able to play both the Modern Benoni and the ed5 Benoni. The Classical Variation of the Modern Benoni offers good counterplay for Black in more than one way, but I prefer the White side of the ed5 variation even if with best play Black can emerge out of the opening with equality. Black can try to sidestep the issue with 8...Re8!?, which would be my recommendation for someone wishing to play 6...c5, but after 9.Nd2 Na6 10.Kh1 Nc7 11.a4 or even 11.f3!? I think White is slightly better. A key motif for White is to play a well-timed de6 to force a positional concession from Black. 

Conclusion: 6...c5 is not a good practical choice unless one has the Maroczy Bind and the Benoni in their repertoire. One must know a significant amount of theory to play this line as Black, and even after this White should be better. So 6...e5 is a more suitable option. I play 6...c5 myself, but only as a surprise weapon.