I think it is important to stress that people writing a book about the Blackmar-Diemer Gambit will adopt a very different point of view than people studying the theory of the Dutch defense.
The Blackmar-Diemer Gambit, in spite of having some hardcore aficionadoes, is a dubious opening. When you study it or write a book about it, you are looking for lines where White can get some initiative and is not clearly worse. Hence, a tense position with some pitfalls for Black or an unclear position with some compensation for the pawn is exactly what you wish for.
I would say the position after 1.d4 d5 2.e4? de4 3.Nc3 f5?! 4.Bf4!? is exactly that: White obviously has some compensation for his pawn with a lead of two moves in development and Black being slightly weakened by f7-f5. My bet is that the objective evaluation is about equality, but one could also argue for a slight edge for either side - I haven't seen Scheerer's analysis but in any case he has no reason not to be enthousiastic about White's perspectives here. At the very least, since there is no forcing play going on, it is pretty sure that Black doesn't have a direct refutation.
Playing Black, I would never consider 3...f5 here: there are so many easier ways (3...Nf6, 3...e5, 3...Bf5) to handle the BDG, with good prospects for an advantage, that entering such an unclear line is totally unpractical.
In the Dutch, by contrast, Black is often playing with some preparation (otherwise it is too dangerous) and he is pretty happy to reach an equal or double-edged position. That's why, from a theoretical point of view, 1.d4 f5 2.Nc3 d5 3.e4 de4 4.Bf4 is not met very often. White has many other options to try and get some initiative, e.g. 3.Bf4 immediately.