A military victory isnt always the clear defeat of an enemy opponent. It can be a subtle affair, where the avoidance of total defeat is all that can be reasonably expected.
in the case of Dunkirk, the Germans had already won their victory, effectively, and all that remained was the capture of the Britih forces trapped in the pocket. Dunkirk was a victory, because it was a plan, successfully completed, that managed to avoid that outcome. In this the British had managed to to achieve their objective....to save as many of the personnel as possible. In fact, they greatly exceeded all expectations. It had been planned that when the operation began that perhaps 25000 men might be saved. When it ended, 330000 men had been rescued. The loss of the equipment was never a major issue for the British, it was not their objective to save this.
From the enemy perspective it was also a defeat. The stated German objective had been the total destruction of the pocket, and the death or capture of its personnel. They failed to achieve their stated objectives
Once the decision had been reached to effect an evacuation, the plan and its execution were done brilliantly. The on ground situation as of 26 May, when the operation began saw the british strung out some distance from the beaches. The withdrawal to the beach head was executed brilliantly and flawlessly, and managed to absorb the major shock of the Belgian surrender two days later......an event carried out without warning by the King of Belgium. The holding actions at Boulogne and Calais, from the 19 May to the 26May were criticasl to the defence of Dunkirk, and were the events principally responsible for the German defeat, whose objectives at that point had been the immediate and early capture of all the channel ports, so as to prevent the evacuation that eventually developed. As Churchill stated, in his cable to the commander of the defending force (principally 30 Brigade, under Brigadier Nicholson), although it arrived too late to be received...."Every hour that you continue to exist is of critical importance to the BEF....Government has therefore decided that you must continue to fight . Have the greatest possible admiration for your splendid stand. Evacuation will not (repeat not) take place".....but even this was not fully observed, as two small motor yachts (the Conidaw and the Gulzar) managed to sneak in and rescue about 200 survivors after the surrender, right under the noses of the Germans......Calais had cost the BEF 3500 men, but had gained 3 precious days for the defence lines behind Gravelines to be organized, and the evacautuion from Dunkirk to be commenced. Churchill later wrote "Calais was the crux to our success. Many other causes might have prevented the deliverance of Dunkirk, but it is certain that the three days gained by the defence Calais enabled the Gravelines line to be held and thereby enable the withdrawal to be carried out.