I think that is what they mean with the ?_escaped_fragment_= URLs: they defined a way to map the #! URLs to URLs that could be sent to the server to get a prere

I think that is what they mean with the ?_escaped_fragment_= URLs: they defined a way to map the #! URLs to URLs that could be sent to the server to get a prerendered page. But I may have guessed wrong, I don't really know.