I’m assuming he measured time by averaging on 100s of instances, or he maintained the exact same input for both versions of code. Would be a big oversight if not!

Both versions use the same input data. I also tried different random initial values and got essentially the same result. I didn't test hundreds of inputs, since that would have been mostly a waste of time in this case. The algorithm and the data distribution remain practically the same. What I'm measuring is the machine code that Clang generates for the hot loop.