That’s only part of the story. The German military at Dunkirk was stretched to the limit. It’s planes, troops, and tanks were worn out. German supply lines were stretched. Their pressure on the Allied forces wasn’t the whole package but it was all they could muster without an all in effort.
Beg to differ. The Germans had faced no real opposition in their drive across Belgium and France. The Luftwaffe has control of the air and most German generals wanted to finish the British off.