I didn't actually read all the comments. I would suspect he feels this way because I bet its much easier to train an unweaned chick not to fly away than an older bird that is already flying, not familiar with the freedom of the outdoors and weaned by someone else. If you hand-feed it yourself and have it outdoors frequently during the flapping and hopping stage and while the bird is still food dependent, then you can do recall training WAY easier. I have baby birds that have recently fledged flying to me and begging for food all time. An already weaned or adult bird, its WAY harder. Plus, with a bird that was weaned by someone else, its going to take a few weeks to get a good bond going. So you aren't going to get the bird flying across the room every single time on recall on the first night home.
The only pet bird I've been able to consistently do recall training with was trained basically like you'd train a passage hawk. Basically, you get it hungry enough so its desperate to take that first jump to the hand. If not hungry enough, you can stand 2 feet away and the bird will lean this way and that and act like they just don't understand the concept of flying to the hand for food. This from a bird that otherwise has been fully flighted and will gladly fly from point A to point B in the living room. Ask it to come to your hand when called and it seems to be a different deal for them. The only time I've really recall trained a parrot was more on accident when it was just on a diet for its own good. Daffy my quaker was just way too fat and headed for liver disease so his food was being tightly rationed. He was pretty hungry. I'd previously tried to recall train the bird with clicker training and it came to me when it felt like it. After the bird is trained and jumping to the hand, then you can start increasing distance and once you get instant response under a variety of circumstances, then you can start increasing food rations so the bird doesn't feel famished all the time. That is how falconers do it. With a parrot, by the time you've done all this and you've conditioned it to really consistent recall, you might be able to do away with weight management and food rations because the bird is bonded to you and wants to be with you. Probably would work especially well with cockatoos and macaws if I were to hazard a guess without reading a bunch of Chris Biro's stuff. Although even then I bet there would be significant variation from one bird to the next.
If you get a chick that hasn't fledged yet and still has that instinctive need to hang out with mom and dad for food, then you don't have to mess around with keeping the bird hungry. In fact, I'm under the impression this is one of the reasons some falconers choose to fly imprints as apposed to already fledged wild caughts or parent raised birds. In case anyone was wondering, I'm reasonably confident that hawks imprint differently than parrots, so while there might be a small bit of overlap in terms of initial training for recall, after the bird has been weaned for a bit, a parrot is going to act like a parrot regardless of who raised it. This shouldn't degrade into a discussion against hand-feeding.
I still disagree with selling unweaned chicks in most cases. The average person just doesn't know how to handle and prevent medical problems or how to wean a bird correctly. And yes, he is right I suspect that many people wean them wrong. Unfortunately, he seems to be under the impression breeders wean them wrong in a sterile environment and pet owners are going to do everything right. Now, lets think about this. Give the bird to someone else that has never weaned a parrot before and may not have even handled a parrot before. Talk about a recipe for a poorly weaned bird. Also possibly a dead bird. I suppose if I specifically wanted to fly a pet bird outdoors, I'd probably want to get an unweaned chick. However, being as I'm one of those people that free-flies the predatory birds that would like to eat a parrot, personally I'd rather not have my parrots flying loose. Having to chase off bald eagles and haggard hawks coming in to harass my falconry bird doesn't thrill me. I'd be in a state of panic if I had to do that for my pet. Personally I'd rather have my pets enjoy their outside time either on a harness or in an aviary. Plus, being as our much loved pets are a non-native species, I personally think they shouldn't be flying loose anyway. My hawk is a native species. If she decides not to come home with me some day, she can just resume life as a wild bird and its not a disaster.