I think immersion is a complicated conversation. I'm pretty sure we all know it when we feel it, but that's very different than saying that we're all feeling the same thing. Personally, I liken it to the experience of reading when you forget that you're reading words on the page. That's not exactly right, but its close enough for a rough description. I would agree that mechanics, in some cases, are a part of RPGs that interfere, or at least can interfere with 'immersion'. That said, I've played crunchy games where I've gotten the immersion feeling, and light games where I haven't, so I wouldn't argue that it's solely a mechanical weight issue. I suspect that it has to do in large part with the group gaming experience, with having the whole table on the same wavelength so to speak, but that's just personal experience.
If you'll forgive me a simile, I liken the idea of immersion across a session to a rock skipping over water when you throw it from the beach. The rock striking the water are the mechanical moments, and the varying arcs in between are, hopefully, at least somewhat immersive role playing.