If you go to the broiler houses they still produce birds in less than 5 weeks we have a bunch around here and you can't tell me that a chick at 5 weeks old hasn't been modified some how I've never seen one yet that does and I know a few people that have broiler houses they get the chicks feed and all and I ask about getting some chicks ? Not allowed!
They have just been selectively bred for years and years to grow as fast as possible and plus they have food in front of them constantly 24/7 and don't move at all. It's no wonder they grow so fast. That said, it's possible they may use some supplement or something similar to hormones and antibiotics that is legal. Or they may use them illegally. But most don't.
And you can buy the chicks from hatcheries