The bit I highlighted is correct. At Dunkirk, the Germans did not achieve their objective; the British did. If we go by simple criteria such as that, it's clearly a British victory. The single and artificial criterion of whoever-controls-the-battlefield to determine victory does not apply at Dunkirk nor at many other places.
While it may be going too far to say Dunkirk was a British victory, it is going equally too far to claim it as German victory. Initially, Germany hoped to place the Allied armies at a strategic disadvantage with Fall Gelb; their unexpectedly wild success led them to modify their objectives to include the conquest of France and peace terms with Britain. They accomplished the first and not the latter.
I recommend re-reading theprestige's comments about Clausewitz and the value (and necessity) of the pursuit. While Clausewitz is somewhat limited in that his framework is quite distinctly Napoleonic style warfare and its constant quest for a battle of annihilation, it is rare to find a military operation in which his words do not offer insight. The Battle of France, The Battle of Aras, the French and British defense of the Dunkirk perimeter, and the successful evacuation of the BEF, even sans equipment, could be put in the original text of On War as the perfect example of what Clausewitz says not only of the necessity of pursuit but of its difficulty.
