I think he knows it was the Vax. The NFL probably insisted that he not state that it was or he would be suspended forever. The NFL would face $$$$$$$$$$$ lawsuits if he confirmed that it was caused by the vax as the NFL required that everyone get one. JMO
Early on, within a day or so, one of the NFL mouthpieces said they would take care of Hamlin “for Life.” So, Yeah, he might not want to risk that assistance since he was not yet a 3-year vested player. The lawyers will kick in after the autopsy.