One particular sentence in that plan is often quoted as proof that the German military leaders knew that the Soviet Union had no intention of launching an offensive against Germany. That sentence reads: "The Russians will not do us the favour of attacking".
It is useful to look at the context of that sentence, to see what Marcks really meant.
The introduction of the "Draft Operational Plan East" reads (Source: Barry Leach, "German Strategy Against Russia 1939-1941", Appendix I, pp. 250-254):
The purpose of the campaign is to strike the Russian Armed Forces and to make Russia incapable of entering the war as an opponent of Germany in the foreseeable future [my emphasis]. In order to protect Germany against Russian bombers Russia must be occupied to the line lower Don - central Volga - north Dnieper. The main centres of the Russian war economy lie in the food- and raw-material-producing areas of the Ukraine and Donets Basin and the armament industries of Moscow and Leningrad. The eastern industrial regions are not yet productive enough.
Of these areas Moscow constitutes the economic, political, and spiritual centre of the USSR. Its capture would destroy the coordination of the Russian state.
As is obvious, Marcks, reflecting the view of the German military leaders, regarded an invasion of the Soviet Union, as essentially a defensive act, to prevent the Soviet Union from attacking Germany at some time in the near future. His plan is a contingency plan, for implementation if circumstances require it, ie if the Soviet Union shows signs of entering the war as an opponent of Germany, ie attacking Germany.
Marcks goes on to describe the war zone. Then comes the crucial part:
The Russians will not do us the favour of attacking. We must expect that the Russian Army will remain on the defensive against us and that only the Air Force and the Navy, namely the submarine arm, will attack. Russia will wage war by means of a blockade. For this purpose a Russian breakthrough into Rumania seems probable, in order to deprive us of oil. At the very least, strong air attacks on the Rumanian airfields must be expected.
On the other hand, the Russians cannot avoid a decision as they did in 1812. Modern armed forces of 100 divisions cannot abandon their sources of supply. It is to be expected that the Russian Army will stand to do battle in a defensive position protecting greater Russia and in the eastern Ukraine. It will find a good defensive position on the line Dvina as far as the Plozk - Beresina - the Pripet Marsh - Zbrutsch - Pruth or Dniester. This line is already partly fortified. A withdrawal to the Dnieper is also possible. In front of this line the Russians will probably fight delaying actions only.
It is obvious that Marcks is here assessing the Red Army's reaction to a German invasion. Since the German strategic aim was to prevent a future military threat from the Soviet Union (a probability given Soviet expansionism in Eastern Europe), the aim of any invasion was to engage the Soviet forces and destroy them at the outset. What might frustrate that aim would be a rapid withdrawal of the Red Army into the interior of the Soviet Union, out of German reach, where it would continue to constitute a threat. The aim of Marcks is to determine whether the Red Army will stand and fight, enabling the German Army to destroy it, or whether it will retreat, as the Russian Army did in 1812.
Marcks' conclusion is that the Red Army would try to avoid an engagement with the invading German forces, in order to preserve itself; that is what he means when he says "the Russians will not do us the favour of attacking". He concludes that while the land forces would try to avoid engaging the German invaders, the air force and submarine force would launch attacks. He sees the main Soviet reaction not as a counter-attack on the German invasion force, but as an invasion of Rumania to cut off Germany's oil supply.
However, he also concludes that the Red Army would not be able to retreat, because it would not be able to leave its sources of supply. Therefore it would stand and fight, implying that Germany would therefore be able to achieve its aim of destroying it.
The words used by Marcks do not mean that the German military leaders did not see the Soviet Union as posing a military threat, as German leftist historians falsely interpret them. In fact, his introduction to his study show explicitly that a Soviet attack on Germany was seen as a distinct possibility, one that might need to be prevented by a prior German attack on the Soviet Union to destroy it as a military threat.