In the regular cut, the voices are fine because The World Between Worlds has already been established but any pre-OT Jedi coming back as physical ghosts doesn’t fit with what we’ve seen (excluding Qui-Gon and possibly Ahsoka).
Interestingly enough, the novelization pretty much flat out tells the reader that the voices are coming from the World Between Worlds. Which does mean it’s different than Force ghosts. More has to do with potential realities, pasts, and futures.