Is it only the acid produced by the stomach that causes heartburn?
Not only the acid is responsible for damage in the lining surface of the esophageal lumen. Other factors include: Pepsin, the digestive enzyme also produced in the stomach that digests protein in the diet; Bile acids, detergents excreted by the liver into the small intestine to digest fat; and proteases, digestive enzymes produced by the pancreas that further digest dietary protein.
Pepsin significantly damages the esophagus.
Bile acids also can cause damage especially when acid secretion is suppressed.
A mixture of acid and bile reflux was found to be more aggressive to the esophagus than acid alone.
These contents of the small intestine reflux into the stomach and then up into the esophagus.
It's important to counteract the effects of these chemical factors by various measures in order to achieve acid reflux relief.