“...could not reasonably have known...”
It is assumed that the player knows the rules, so he reasonably should have known that he broke one of them.
He said he didn’t know he had violated the rule Couple that with the fact that the Rules Committee reviewed it while he was still playing and said there was no violation, thereby issuing a ruling, it was unknown by anyone when play concluded that there was a rules violation making and incorrect scorecard possible.
They ultimately got the ruling exactly right. Two shot penalty assessed on 15.