After 1.d4 d5 2.e4 dxe4 3.Nc3 e5 4.Nxe4 Qxd4 5.Bd3 Nc6 6.Nf3, Scheerer suggests 6...Qd5 7.Qe2, indicating that 7.00 Bg4 8.Nc3 Qd6 9.h3 Bh5 10.Nb5 Qe7 11.Be4 a6 12.Bxc6+ bxc6 13.Nc3 f6 is better for black, as in the game Leisebein-Lange, 2004.
Even if black won this game, this position does not seem a clear cut. After 14.Re1, black has following lines :
a/ 14...Rd8 15.Qe2 and now
a1/ 15...Qf7 16.Qxa6 Bxf3 17.gxf3
a11/ 17...Qe6 and Houdine gives me 7 possible moves with complete equality
a12/ 17...Ne7 18.f4 with complete equality, eg 18...Qg6+ 19.Kh2 Qxc2 20.Be3
a13/ 17...Qg6+ 18.Kh2 with small white advantage
a2/ 15...a5 16.g4 Bf7 17.Bg5 with small white advantage
a3/ 15...Bxf3 16.Qxf3 Qe6 17.Qh5+ with small white advantage
a4/ 15...Qc5 16.Be3 with white advantage
b/ 14...Bf7 15.Bf4 with small white advantage
c/ 14...Bg6 15.Bf4 with small white advantage
d/ 14...a5 15.Bf4 with equal game
So bottomline : I cant see black's advantage in the posiyion after 13.Nc3 f6. In fact, black is struggling for equality. So don't believe everything you read in the books - even the assessment of "experts" is often flawed. Do your homework and assess the position yourself, using your style as the ultimate guideline,
There are in fact authors who claim they can produce a chess book or DVD each day ! Needless to say they are also have a distinct opinion on the Blackmar Diemer...