The Firebird
Commoner
What 'actually is' here that's being changed???!!! There's no real runes, there's not even a fiction of what these pretend runes 'mean'!!! So, nothing the character did, fictionally changed anything except their knowledge of the runes' content. That seems entirely diegetic to me. In the real world all that changed is the fiction was updated to add an agreement about the runes' content. This new fiction is in accordance with the wishes of the player who rolled the dice. This is literally no different from an attack roll in combat.
In character, it is reasonable to change things happening now or in the future, but not in the past. So the character's hopes changing fiction which ocurred in the past (when the runes were placed there) is nondiegetic. While their actions changing present and future (whether the orc lives) are.