The device is built on known science, and the brain may learn the response, but it is in response to a stimulus. Without the stimulus, the brain does not increase increase delta power on it's own.
There will be other discoveries in the future, who knows what we'll learn, but the current body of knowledge (and there is a decent amount of it) does not suggest a learned response to be the case.
Furthermore, the sleep we stimulate is deep sleep, which naturally degrades as we age, which perhaps suggests that it isn't a learned response, or else why would we see a reduction with age.
There will be other discoveries in the future, who knows what we'll learn, but the current body of knowledge (and there is a decent amount of it) does not suggest a learned response to be the case.
Furthermore, the sleep we stimulate is deep sleep, which naturally degrades as we age, which perhaps suggests that it isn't a learned response, or else why would we see a reduction with age.