Edit: someone asked how this works..
1) it looks up the resource in the mapping (linked above) (matching the cdn and file path).
2) if found, it replaces it with the copy it includes:
So for those files, requests are never made to the CDN.
If the website uses a different CDN; a lib not recognized; or a version not recognized.. then the request is still made.
Last-Modified/If-Modified-Since is an optimisation trick which exists for the situation where the person running the website hasn't bothered to explicitly define expiry periods for content.