I don’t necessarily think we can assume that Walt’s appearance means that he’s there on the island again as a real, physical person. So far he’s no different than any of the other appearances of people on the island who may or may not be real and/or dead.
Heck, with Walt we even have precedence for him appearing when he wasn’t really there, and you also have precedent with Locke seeing and interacting verbally with people who have formerly moved on (such as Boone).
I think it’s rather obvious that it’s Jacob and not the actual Walt. But you do make me think about how Jacob/island appears.
The people that don’t have that special connection can’t communicate with it. The only person we know that can talk to these manifestations are Ben and now Locke.
Now Locke really connects.
So you are thinking that the Walt “thing” is the same person/thing as Jacob? I’m not sure about that yet.
I definitely do not think that every time we see other people appear (Eko’s brother, Jack’s dad, etc.) that it’s “Jacob”. It’s possible, but I don’t think we know anywhere near enough to do anything but guess about that.