There’s no question that Hiss was guilty of treason. Daniel Patrick Moynihan (if memory serves) managed to get the so-called Venona Files de-classified. He’s clearly mentioned in them as “Ales” - and acronym of “Alger Hiss.”
Hiss avoided talking about his specific case and instead spoke in generalities about current foreign policy. When asked by a questioner about his case, Hiss went on about how the evidence they got on him was acquired illegally, blah, blah, blah.
Hiss never came out and said "I was framed!", instead, he used lawyerly evasions to defend himself.
I walked away from that lecture convinced that Hiss was guilty of spying as accused.