Yes, closing your sinuses is a natural reflex response for humans, and people have greater or lesser at will control over it.

The nose holding for swimming is more about how strong that sinus closure is and endurance. People with larger sinus openings have a more difficult time keeping them closed and resisting pressure like water entering from jumping into a pool. Also some people have a hard time keeping them closed for any prolonged period.

In other words, you just have totally ripped sinuses breh.