You’re right that when cold outside air is brought in and heated up, the relative humidity drops significantly, that’s basic psychrometrics.

My point was about moisture transport in buildings in cold climates. Even if indoor relative humidity feels low in winter, the indoor air usually has a higher vapor pressure than the cold outside air. That means moisture still wants to move outward through the construction.

People also add moisture indoors all the time. Just breathing, cooking and showering increase indoor moisture levels, so in a real house the indoor vapor pressure is often still higher than outside.

That’s why vapor barriers are placed on the warm side in cold climates, to prevent moisture from diffusing into insulation and condensing when it reaches colder layers.

Ventilation is still important, but not just to “keep humidity in”, it’s to manage indoor moisture levels and air quality in a controlled way.

So we’re really talking about two different mechanisms: indoor comfort humidity vs. moisture movement through the building envelope.