The Short Version
The claim is largely accurate. Multiple veterinary and canine dental sources confirm that the abrasive felt on tennis balls — especially when contaminated with sand or grit — can wear down enamel and dentin in dogs, producing flattened crowns and clinically meaningful dental damage. The term "tennis ball mouth" is used informally to describe this condition, though it is not a standardized veterinary diagnosis. The main caveat: significant wear typically occurs in dogs that chew obsessively or for prolonged periods, not from occasional fetch play.