Welcome to the gastroenterology community! I have never heard of smoking increasing gastric emptying. Where did you hear this?
One of the active ingredients in cigarettes is nicotine. Many of the neurotransmitter receptors in the stomach are a variety known as acetylcholine 'nicotinic' receptors. Depening upon the pathway these receptors are linked to, it's possible with a fully functional neural system in the stomach and upper GI tract, the result could be increased motility.
From my research, I found that smoking may increase gastric emptying for liquids but it slows down gastric emptying for solids.