You are technically correct. However, Hiss did serve as the secretary-general of the United Nations Conference on International Organization (the United Nations Charter Conference) in San Francisco in 1945.
For someone to say he was "UN Secretary General" is more of a slip, or a memory lapse, then a purposeful deception. Your reaction is a tad harsh for a minor error.
I did post a line from the UN website stating who the first Secretary General was, and you are correct, that rebooted is technically correct. What I did find interesting in reading about the creation of the United Nations - from the UN website - is that Hiss’s name never appears. One would have to wonder why.