The difficulty is that, by a suitably chosen narrow meaning of defeat: Destruction of the army in the field, loss of all conquered territory, or significant loss of hoe country areas, the German Army wasn't defeated. Here is a map that shows the Allied March to the Rhine after the Armistice; which thus shows that on Nov 11, 1918, the Germans still held all of Belgium east of the line Ghent-Maubeuge; significant French territory between Longwy and Metz; and all of Alsace-Lorraine.
Given adequate supply, logistical and reinforcement support from the home front, the German General Staff position that the war could have been continued is reasonable.
Of course, we know now that that assumption was invalid; the Allied blockade had had its desired effect, both the German economy and home-front morale were completely broken, and further continuance of the fighting was moot, leading only to an inevitable destiny; but the German army, and particularly its senior commanders, had been sheltered thus far from the privations of the populace.
It is thus a curious paradox, for which I cannot recall any earlier precedent in history, that while the German Nation had been utterly defeated, its Army had, in some sense at least, not been defeated.
Update: from here (my emphasis):
By mid-October Ludendorff and Hindenburg wanted to distance themselves from a negotiated settlement or surrender to Allied demands. Now politically powerless, Ludendorff favored what he could have done better had he not gambled on his offensive and ruined the moral and fighting strength of his troops: he was for fighting on the defensive – through 1919. He and the German admiralty insisted that submarine warfare continue. And on October 20, Hindenburg told Prince Max that his government should keep Germany fighting "for our honor to the very last man," and if this resulted in Germany breaking off negotiations with Wilson, claimed Hindenburg, so be it.