Done properly, that's not terribly true. Done properly, correct code transformers can actually bring you a lot of power, such as handling exceptions correctly across asynchronous boundaries or enabling much more powerful control flows, such as being able to freely pass in callbacks that themselves may have asynchronous functionality, layered arbitrarily deeply. The only thing that can "leak" is exactly where the asynchronous breaks occur, but it's not that hard to see that.
It's quite debatable that "the true nature" of asynchronous code is in the manually-scheduled hacked up code that you write by hand in Node.js or similar tools. Other environments that don't require that make a good case for saying that the "true nature" of asynchronous code looks like the synchronous code, only with capable scheduling, and the real "leaky abstraction" is the way your terrible, terrible language runtime scheduler is poking out and requiring you to manually transform your relatively simple code into a spaghetti mess to satisfy it. There's your leaky abstraction, and what leaks out is daggers slicing your code to ribbons.