German combat performance at the tactical and operational level in WW1 and WW2 was superior on average to those of other countries

I'd certainly take issue with him there; at the tactical level the the French had already arrived at effective infiltration based squad tactics by 1915, and had codified these by 1916. It was only with the beginning of 1916 that the Germans began to experiment with their no doubt effective, but ultimately flawed, 'Stosstruppen' Units. The British took somewhat longer, but by the beginning of 1917 had settled on a system similar to the French, placing them on an equal level with the enemy. For WWII his argument may have some weight, especially with regards to the use of the MG 42 by German infantry, but allied small unit methods were adequate, and no less effective. The quality of the German infantry declined in both wars, rapidly, from 1916 and 1942 onwards, and this played a role in declining effectiveness at the tactical level, obviously.

As to the Operational Level, while the Germans acknowledged the Operativ, it seems to have been completely ignored. From the Invasion of France to Gorlitz-Tarnow, to Caporetto, Romania and the 1918 Spring Offensives, the Germans sought 'battles of annihilation' that, while they made impressive gains and inflicted great losses, proved incapable of delivering strategically valuable results. 1914 ended with a two front war and c. 800 000 German casualties in the west; Gorlitz-Tarnow ended with 11th Army having suffered close to 50% casualties, and Russia would remain in the war until the beginning of 1918, while the eastern front tied down valuable forces; Romania survived 1916, and while it to surrendered in mid-1918, it's army was able to mobilize for Nov. 10th, and then go on to fight in Hungary; Italy was badly shaken at Caporetto, but remained in the war, and struck a deathblow to the Austro-Hungarians at Vittorio Veneto in 1918; finally, Operation Michael and it's sub operations decimated the 'stosstruppen', who were assigned no strategic or operational objectives, while failing to cripple the BEF, and leading to German reverses before the Americans had even begun to arrive in significant numbers at the front.

While the Allies displayed similar insufficiency at the Operational Level, by 1917 Ferdinand Foch had arrived at a method, Bataille General, that presaged future Soviet Deep Operations, and which had the Western Front armies on their knees by November 1918. Had the allies advances continued past the Scheldt and into Alsace-Lorraine, the German Army probably would have disintegrated.

In WWII it was more of the same; impressive initial gains, against poorly lead and organized foes like France, Poland, and the Red Army in 1941, but the strategic picture from December 1941 onwards never changed: Germany was at war with the foremost superpowers of the World, and conducting costly operations on a logistical shoestring. Only by plundering the occupied territories and resorting to measures like forced labour were they able to survive, while Allied operations in Italy, Normandy and on the Eastern Front could only be delayed in their ultimate result: the destruction of the Wehrmacht and the destruction of the Third Reich.

Purely my opinion, though I'm sure some might agree, while the Germans might claim a degree of tactical superiority in both wars, it was their failures at the Operational and Strategic levels that made their defeat all but inevitable. Not a glowing, in fact quite a damning record, for the fourth of Europe's so-called "perfect institutions", the German General Staff.

This superiority is attributable to the structure of the German General Staff

The General Staff, or at least the higher command structure, did play a role in the formulation of tactical doctrine, but as stated, it's failures at the Strategic Level, ie involving Germany in lengthy wars against superior foes, precluded that tactical 'efficiency' from translating above the Operational Level.