Ah that totally makes sense and seems like it's Apple's way of tacitly acknowledging that people around you can hear the built-in speakers. With the immersion level turned down it seems like the noise cancelling AirPods will be a bigger hurdle to someone getting your attention than the headset itself. (putting aside the silliness you'll feel talking to someone while casually wearing it)

They also announced new feature to airpods something liked auto adaptive immersion that they detect what to noise cancel and what not dynamically depending on changing environment. On video they showed someone when someone start talking to you they will reduce immersion and the same when you start talking.

Similarly when when showed up close to you while you are in fill apple vision pro VR impression mode they will reduce it see this person slightly blended with your VR world