The 15% is the average of 5 games shown in AMD's slide.
10 Games, two slides. Same 15% increase.
So far these are the lists of games with 15%+ performance improvements(3D-V Cache) over a stock 5900X and 12900KS Alder Lake. Pay attention to the Tie/Even games where it does the same with the 12900K so it must be a frame cap or something. Also CS:GO Three of the CPUS ties, the 5900X, 12900K and 5800X3D so the issue is on the game.
DOTA 2: 18% Over 5900X - No info on 12900K
Monster Hunter World: 25% over 5900X - No info on 12900K
League of Legends: 4% Over 5900X - No info on 12900K
Fortnite: 17% Over 5900X - No info on 12900K
Final Fantasy XIV: 20% Over 5900X - 20% Over 12900K
Shadow of the Tomb Raider: 10% Over 5900X - 10% Over 12900K
Far Cry 6: 20% Over 5900X - 11% Over 12900K
Gears V: 12% Over 5900X - Tie/Even with 12900K
Watch Dogs Legion: 40% Over 5900X - Ties/Even with 12900K
CS:GO: Tie/Even with 5900X - Tie/Even with 12900K