Wow... I always thought this scene was legit.
How Charlie Chaplin accomplished the roller skating stunt in "Modern Times" - Album on Imgur
But truthfully, I'm not disappointed... I think this is just as amazing. I've been aware of matte painting for a long time... just never knew it was used in this scene.
For those who want to know more:
https://en.wikipedia.org/wiki/Matte_painting