I'm also quite surprised that he said the irony is that these songs are not in emotionally satisfying shows. I find Sondheim's shows among the most emotionally satisfying musicals ever written. Happy, maybe not. Resolved, maybe not. Satisfying? What does it mean? I am satisfied that my emotions are engaged in a deep and layered way. I don't walk away from Follies happy or resolved that everything is fine but I definitely feel more in that show than in most shows. Even Assassins engages me emotionally and I'd say in terms of character and story it's one of the most intentionally unemotional musicals there is. Even the absurd, stylized and satirical and therefor mostly unemotional Anyone Can Whistle ends on an emotionally affecting song.
To say something isn't emotionally satisfying to me is like saying it leaves you cold or doesn't engage. These are things I couldn't say is true of Sondheim's work ever.