Actually, what the site says is air guns can diesel, and in the past some makers recommended adding oil to enhance the effect, but with modern construction materials and lubricants they shouldn't diesel. It goes on to say that with proper lubrication, the dieseling effect is small.
But with improper (excessive) lubrication with petroleum-based lubricants, dieseling can take place, and with enough lubrication it can damage the gun -- and guns have been damaged that way in the past, which was my point.