I doubt it takes 1 year to expand the flight model for atmospherics. Well ideally they'd be working on that since 2019, but I guess they don't have the manpower or budget to do it for this expansion.
Well, we don't know how long it would take them to do atmospheres and oceans plus all required attending features. I certainly don't. I'm certain you don't. Which is why i threw 1 year out there as a possibility.
Maybe it would take them 6 months, maybe 2 years. It took them 2 years to do airless worlds for Horizons (with attending features).