Like all generalisations, the exceptions disprove the rule. In this case does smoking, snorting cocaine or injecting heroin in moderation apply? Certainly not. It probably applies with eating and drinking but what about exercise? Is strenuous exercise good for one’s health?
This maxim is often bandied about as a justification for behaviour that is clearly suspect or injurious to one’s health. Yet those protagonists of its use would never use this saying in other contexts where they disbelieve its veracity.