You have, in fact, lit upon the mechanism at play: some infants are sensitive to even tiny CO2 concentration changes, particularly when risk factors are combine

You have, in fact, lit upon the mechanism at play: some infants are sensitive to even tiny CO2 concentration changes, particularly when risk factors are combined. There is a reflex where instead of breathing more when CO2 levels rise, infants stop breathing all together: https://www.frontiersin.org/articles/10.3389/fneur.2015.00220/full https://www.frontiersin.org/articles/10.3389/fneur.2015.0022... While a car is moving it has ventilation, and it's illegal to leave an infant alone in a parked car in part for this very reason. But sleeping in a car seat is one of the risk factors for SIDs.