Yes, that's true.
During pregnancy the high level of estrogen stimulates prolactin secretion from the anterior pituitary. This increased prolactin causes growth of mammary tissue in preparation for lactation.
However, at the same time the high level of estrogen and progesterone during pregnancy blocks the "milk receptors" of prolactin so that milk production does not happen during pregnancy.
After birth and removal of the placenta and the decrease in estrogen and prolactin there will be no more down regulation of these prolactin receptors and milk production can start but this time it has to be stimulated by baby suckling. If no suckling milk production stops.
Oxytocin is also stimulated by baby suckling and it aids in milk let down (ejection) not milk production.