Your first point is correct. There are weather decks along the sides and aft of the ship, underneath the flight deck, where crew can be outside.

But these sailors aren’t working 24/7 non-stop. Individuals have downtime to sleep, eat (obviously), and generally hang around playing video games, watching movies, reading, etc (this one is not so obvious). They would’ve had to bring an that stuff onboard before they left.

Regardless, that’s like being stuck in your house for 9 months without ever touching grass. It’s just not sustainable psychologically.