Let’s go back to the definition we were discussing:
From your response above, “gaining an advantage in the game” includes “if the player thinks that their alteration to the world makes the game more enjoyable”.
So, a player is “altering the game reality” if the player is given authorial power to establish a prior event or detail for the purpose of making the game more enjoyable.
In other words, the concern isn’t “a player taking advantage of the system as an “I win” button”, the concern is “the player exercising authorial intent that the DM believes is rightfully his”.
Yup. That’s exactly my point.