Simple terms. Immersion is a state of mind, a kind of 'leave your hat at the door and get on the ride'. It is deceptive, geared towards making your body believe something it knows is false.
As immersive interfaces become more capable of approaching realistic environments, that's when things get blurry.
Immediacy on the other hand is pretty direct. It is totally experiential. It has no emotion trickiness about it. You either experience it or you don't.
Immersion can make it easier to get to. So can simple, uncluttered interfaces. But even in the most hypermediate display in the most hypermediate world, immediacy can still persist, through something as simple as shock, or pain.
Not the most pleasant experience, but immediate (and in some ways immersive) nonetheless.