It's not a major error at all, and I thought it was pretty obvious.
Here is what we know:
Anakin is never taught at any point that Jedi can disappear after death. he does not know it can happen Therefore, JEDI are not taught this. This is because it NEVER HAPPENS!
If it has happened before, it is in the depths of unrecorded history.
We know that RM has said that Anakin disappeared after death.
We know that GL has said that Jedi do not normally disappear after death, and we will find out why the three in the OT do in Episodes II and III.
We know that Anakin does NOT know about the Ghost form. We know that, therefore, this is also something else that will be discovered in Episodes II and III. I reckon it is obvious to assume that ther two are linked- those that disappear, come back as a ghost. MAYBE only visible to Luke; we don't know yet.
If they are not linked it is ludicrous- BOTH mystical things are discovered in the next two films? Unlikely.
How much more bloody obvious do you want it? The implications are crystal clear.