। ॥ १७३ ॥
In plain words
During deep sleep, once the mind has dissolved, nothing exists at all -- everyone knows this from their own experience. So samsara is nothing but a construction of the mind. For this person, it does not actually exist as a real thing at all.
Close to the Sanskrit
At the time of deep sleep, when the mind has dissolved, nothing at all exists -- this is universally established by experience. Therefore samsara for the person is only a construction of the mind; it does not exist for this person in reality.