Working out with a backpack and milk jugs?

Theres a guy I know who has recently became very muscular and he says he doesnt go to the gym or lift traditional weights. Instead he just takes a backpack and fills it with loads of heavy stuff and does push ups with it on his back, he also uses the bag for squats, lunges etc and uses large milk jugs filled with water for bicep curls and arm work. Do you believe you can build a lot of muscle this way?