I’ve worked in and run my own game companies. The request for a slider isn’t based in any kind of misunderstanding about how it would be implemented.

For your example in tetris it would be a global multiplier on the speed. The speed would still increase by the same rate but the actual speed is always multiplied by some constant.

The Tetris speed is already multiplied by a constant anyway even if the difficulty isn’t exposed. And this constant has to be picked by a designer. All I’m asking for is to expose it with a slider. There is pretty much always a constant like this in any game