I don't think he used the 26-1a, replay the last shot, option. He said he moved back and to the left. I don't think the ball rebounded as far left as people seem to think. It is almost impossible to get a real idea of the actual angle from TV. From where he was it probably looked like it rebounded almost directly back at him, just slightly to the left of his original flight line. So under 26-1b he moved slightly left and back as far as he wanted to.
The reason I think this is the case is because he specifically said that he moved to the left. Even if he was somehow conflating the relief options under 26-1a and 26-1b, as some are claiming, moving left would not have fit. The only way moving left fits is if he did so to put the last point where the ball crossed the margin of the hazard between the flagstick and the drop.
And IF that is the case then he is fine and there is no penalty EVEN IF it is later proven to have been a drop in the wrong place. That is because of Decision 26-1/17. If the player made an honest judgement of where the ball last crossed the margin of the hazard and plays his ball he is held harmless if it is subsequently discovered that he played from a wrong place:
Q. In the circumstances described in Decision 26-1/16, what is the ruling if A, having dropped a ball in a wrong place, plays it before his error is discovered?
A. A must continue play with the ball played from a wrong place, without penalty. Applying a penalty under Rule 26-1 for playing from a wrong place (see Rule 20-7) is not appropriate. Otherwise, a competitor would risk incurring a penalty every time he makes an honest judgment as to the point where his ball last crosses a water-hazard margin and that judgment subsequently proves incorrect. (emphasis added)
While the decision deals specifically with a drop from a lateral hazard, the principle should apply to any drop from a water hazard and, in fact, the bolded portion refers to water margin hazards in general, not just laterals.
For context, Decision 26-1/16, which is referred to in the above decision says:
Q. In stroke play, A's ball goes into a lateral water hazard and is not found. A uses his best judgment in determining the point where the ball last crossed the hazard margin. B, A's marker and a fellow-competitor, agrees with that judgment and A drops a ball in accordance with Rule 26-1c, using the agreed point on the margin as the reference point. Before A makes his next stroke, C, another fellow-competitor, says that A's ball last crossed the hazard margin 20 yards beyond the point judged by A to be the point where the ball last crossed. A's ball is then found where C said it would be. What is the ruling?
A. When A dropped the ball under Rule 26-1, it was known or virtually certain that his original ball lay in the lateral water hazard. Therefore, Rule 26-1 was the applicable Rule and the player proceeded correctly in that he was permitted to put a ball into play under that Rule. However, as he dropped his ball in a wrong place, A must correct the error under Rule 20-6. He must proceed in accordance with any of the applicable options under Rule 26-1 with respect to the correct reference point (see Decision 20-6/2). A is precluded from playing the original ball from the hazard.