It wasn’t “just” the audience. Sweetening enhances the laughing - it doesn’t replace it.
I always thought of it this way. You see fun videos of sitcom outtakes, everyone is having fun on set. This means multiple takes. How hard would you imagine it is for an audience member, on the 15th take, to laugh? Sweetening.
I can recall numerous comedy movies I went to where I was the first to laugh. Like a full “ha” before everyone else started laughing. I am unsure if I influenced others to laugh, I was more satisfied that I didn’t need another person to laugh to actually enjoy what I was seeing.