The original post I made was in reply Invader regarding making more power out of a given engine by adding more fuel.
It is correct for diesels, it is incorrect for petrol engines.
You can't really make a black and white statement like that without more details. A petrol engine running lean, or even at stoich will almosr certainly make more power by adding fuel. Likewise, if a diesel is already injecting enough fuel to producing lots of black smoke, then adding even more fuel just means you get even more black smoke.