First time I heard high density stunts growth, high density orchards I thought were done to increase overall yields, increase early production, and to cut down on labor and liability. Nobody is on ladders. Harvesting is much quicker, and nothing to fall off of. Here in MI we are heading that way. Various systems are being tested at MSU and other places. Pruning is used to keep them small, hence you can plant them closer together. Never seen anything that indicates this will stunt trees. Most are kept small by pruning and rootstock.
Here in MI doing the 2-4 in one hole isn't a good idea. Too many fungal pathogens, they love it, trees are so dense it's like a giant dinner table for them. Closer plantings are fine, but not that close. Also it's a maintenance nightmare. I feel this is more for marketing and selling more trees. In CA it can work, but still requires a lot of maintenance. I think for ornamental reasons it's cool, I would do it if it worked here. I have never seen a commercial orchard employ this technique. They tend to use more traditional high density techniques developed in Europe.