I believe that Phil Galfond was one of the first "new-generation" players to explain the strategy with modern media (youtube, podcasts) in a way that it could be taken and applied in your own game without too much hassle.
Harrington had explained the concept of evaluating tournament situations based on a range (although he didn't necessarily call it a range) in his Harrington on Hold 'Em books.
Galfond's gbucks article, maybe not the first mention of it, brought the concept of equity against a range into the mainstream.
