I didn't say 2500 raters players look 15 moves in advance in games (though in some simple endgames they can see farther than that). I said that they can see 15 moves in advance when they spend weeks (or even years) before the game preparing their openings. It isn't unheard of for players at the >2000 level to pull out a planned novility 15-20 moves into a game - a position never before reached in chess that they prepared for one specific opponent in a tournament.