I would say that only the Instruction set is cross licensed, implementation techniques and hardware interfaces aren't (AMD CPUs shall not be compatible with Intel sockets, for example). Intel's SMT is known as hyper-threading, the patent has expired too but the name "hyper-threading" is still copyright protected and the property of Intel.
Not sure if SMT parents would fall under that, but it was my impression that AMD was relatively unconstrained wrt Intel patents due to this agreement.