Airtight homes doesnt mean no ventilation but no cracks in walls and doors and window framing

In the context of avar's question the active ventilation is a safety system.

A little bit of research indicates that candles are not all that different in oxygen consumption and CO2 production than a human and that a single candle (or person) wouldn't use up the oxygen quickly if the ventilation ceased.

As long as a failure of the ventilation system is obvious (so a window can be opened or whatever), it doesn't seem like it would be a major concern.

Well, a candle isn't enough to be a concern. But state of the art air tight passive houses include ventilator units that exchange air while retaining (or rejecting) as much heat as possible.