I don't think so. Not all yoga instructors have great physiques, it's not a requirement. However, discipline is a big deal in yoga, it's called tapas or fire. So I think a lot of yoga teachers are probably really disciplined w.r.t. their diet. Also a lot of them are generally active people and do other stuff.
But can it lead to hypertrophy? No or at least, nowhere near as effectively as resistance training. The loading via bodyweight simply isn't there.
That said, yoga is a lot more than just stretching. A regular yoga practice will produce a balanced strength in the body, and creates immense body awareness. As such, I feel it's a valuable adjunct to weight training for bodybuilders & lifters. I think it's not well understood in terms of the physiological benefits, otherwise a lot more lifters would be doing it, in fact I think in the future they will, especially powerlifters, if only because it can reduce the injury rate from lifting drastically.