Synchronize Properties in Swift 3 using GCD

Question

I watched this years WWDC GCD talk lately and I think there is a code snippet something is wrong with. It is about making a property thread-safe using D

Answer 1

But i think this snippet is not valid, because the internalQueue could be concurrent

But it isn't concurrent. Dispatch queues that you create are serial by default. That is the point of the technique (and the example).