When you mentioned that active stretching and US didn't work and the tendon very tight felt on palpation, I thought it's worth while looking at the fascia as well as neurodynamics. With the traumatic semitendinosis rupture it's almost a given that the fascia loses its elasticity and most likely will result in restriction to the muscle. I find it very helpful to look at fascial release in combination with neurodynamics, kind of "winding up" to feel the soft tissue restriction. If it is indeed the case, ROM and stretching should improve after.
But I agree with the previous comment on checking all the joints above and below before going into soft tissue techniques.
Good luck.