I think there needs to be a discussion of "mechanics" are so that there can be understanding of what GM fiat is. Because GM fiat is not a mechanic.
In WEG D6, GM fiat isn't just a mechanic, it's the core mechanic:
"As the referee, you have to know the game rules and interpret them during play. The players can have their characters “try” to do almost anything; the rules tell you how to determine if they succeed or fail.
It’s a three step process:
1. Determine how hard the task is and pick a difficulty number.
2. Determine which skill is used for the task and have the character roll their skill dice. (If the character doesn’t have the skill, they roll their attribute dice.)
3. If the character rolls higher than the difficulty number, they succeed. If they don't, they fail."
The GM always sets a difficulty number, and they are free to pick it completely by fiat. And in a system with no auto success or failure on a roll, that means they can create a new reality with every check. Completely re-writing the world this way would be ridiculous, of course. But it would still be following a mechanic.
It's seems to me that this is the second or third time you've tried to separate mechanics from not-mechanics and it's been trivial to point out mechanics for things you claim aren't. Here's the thing: anything can be a game mechanic. All someone has to do is write a rule for it. And in a world with thousands of TTRPGs written over half a century, someone has probably tried it.