One possibility is that Isildur analyzed the current standard of top-level heads-up play and found out that the strategy was exploitable by, for example, playing more aggressively in a lot of cases. If that's the case, standard play will adapt.
Or you could apply Occam's Razor and conclude that, in a game dominated by chance he simply had a lucky run. There is always a lottery winner, it's just that you can't predict who it will be. The early comments on Isildur's play were that there was nothing special about him but that he was aggressive---precisely the kind of player who would best leverage a run of luck.
