Don't fully airtight structures need a lot of redundant safety systems to protect against just leaving a candle on killing everyone from carbon dioxide poisoning?

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.

There are several companies in the US producing prefab Passivhaus homes. Typically, a building membrane is used as the airtight layer. Joints get taped, both in the factory and on site. E.g. http://www.phoenixhaus.com E.g. https://www.ecocor.us/prefab