OK, so first, Dunkirk is marshy, meaning it's a great place for tanks to die. General Rundstedt moved towards Dunkirk, noticed that it was extremely poor ground for tanks, and halted. Hitler backed up that order. The Luftwaffe had literally flown at 100% for 2 weeks straight and was exhausted, and the German armies were simply not ideally positioned for charging into a cornered army that would be literally fighting for its very life. Dunkirk was also not the only fight still ongoing - there were pockets at Lille, Calais, and Amiens as well.

Now, in hindsight, everyone looked back and said this was a mistake. But at the time, I doubt anyone (on either side) would have thought it possible to get 336,000 people out of Dunkirk (whose port facilities had been flattened by the Luftwaffe) under the Luftwaffe's nose and past German artillery. The halt order makes sense if you think that you have time on your side - which the Germans undoubtedly did until everything went wrong.

So by no means was this an instance of Hitler saying "let them go"?