They say in the article that the theory is that the baby is breathing in a pocket of stale air trapped in covers or around the baby, and that the fan going moves the air around.
The air near the mattress may be more than stale, it may actually be poisoned. New Zealand urges parents to wrap mattresses in polyethelene covers to prevent the release of gases from chemicals used in fire retardation. Moving the air, moves the poisons away from the baby. Preventing the release of the gases in the first place with the cover seems to be working amazingly will in New Zealand.