No, they're just putting out enough light to override the background, then showing dimmer areas for black. If they have a display with enough intensity and dynamic range, they can pull this off. Eye contrast is local, not absolute, so this works within the dynamic range of the human eye.
No, they're just putting out enough light to override the background, then showing dimmer areas for black
Right, that's how all existing HMD systems work - but generally not with the whole FOV re-rendered so it's not so cut and dry.
Note that such a system doesn't give you black ever. It gives you muddy [insert lens color on the grey spectrum].
The case you describe ends up with what is practically an opaque lens that is replicating the environment that is not virtual. So you might as well just use VR with camera pass through at that point.
That approach is devastatingly hard, but probably the best way to do it.