How can yoga be a workout?

My cousin Dan is a yoga instructor. He really takes his yoga seriously and he hates it when anyone disses yoga and makes it sound like anything but a thorough workout. I don’t do yoga, but I am a physical trainer. I have been working out for years, since I am a gymnast. I do some yoga to help with my stretching, but I really don’t do the harder moves. I’m still pretty confident that I am in better shape than anyone that just does yoga. I know how to stretch and I have to be limber if I want to be a gymnast. I have to be able to twist and turn into all positions. He can do that and more, but can he lift the weights and do flips and jumps like I do. The other day, he got really tweaked because I wouldn’t tell him that his yoga was a good workout. Like I said, I knew some of the moves, but not all of them. He finally pushed me into working out with him. He brought me a yoga mat and he told me that I had to keep up with him or admit that yoga was all about physical training. I didn’t want to succumb to his bullying. When I told him he was bullying with his pushing me to do yoga or admit that my physical training wasn’t as hard as his yoga, he backed down. I know he still thinks he is better, but I bet he couldn’t stand up under the pressure of a personal trainer.