I want to see that shader. How is sloshing implemented? Is the volume of the bottle computed on every frame?
Clearly, there's some sort of a physics simulation going on there, preserving the volume, some momentum, and taking gravity into account. That the result is being rendered over the shader pipeline rather than the triangle one doesn't make it any more or less "real" than the rest of the game. It's a lie only if the entire game is a lie.
Is it really doing any sloshing though? Isn't it "just" using a plane as the surface of the liquid? And then adding a bunch of other effects, like bubbles, to give the impression of sloshing?
Clearly, there's some sort of a physics simulation going on there, preserving the volume, some momentum, and taking gravity into account. That the result is being rendered over the shader pipeline rather than the triangle one doesn't make it any more or less "real" than the rest of the game. It's a lie only if the entire game is a lie.