I think what he means is that the roots are still growing through the rockwoll, and are not acutally in the water, so seurely they'd dry out and die.
There are two methods of getting around this, one is true bubbleponics, the other is easier to use but more fiddly.
1. you rely on the bubbles that the airstone creates to keep the rockwool cube moist, if you hold your hand over the water, you should feel small bubbles splashing against your hand, the idea is that you raise the water level so that these bubbles wet the rockwool and roots in the net cup, it takes a while to zone in on the correct water level, but once you find that your rockwool is damp, but not sooppiong, but not too dry, then mark off this water level. The bubbles will also increase the pressure in the airspace causing it to mist up also helkping to wet the roots. To control the wetness of the rockwool, you either raise or lower the water level, or create more bubbles with a bbigger pump/better stone etc
2. You have a submerged water pump that pumps water up feeder tubes which perform as drip feeders for the rockwool until the roots have grown into the water. This is fairly straight forward although i'd assume it could be fairly easy to overwater the rockwool, and it's also just more parts to go wrong, get logged etc, with bubbles it's litereally a pump and a stone and that's that.
I#ve only ever used method 1. I see no reason to use feeder tubes and water pumps, just not needed.
