On the flip side of the coin, you do see his robotic lower half in the hologram, therefore people who only recall Maul doing the torso splitz in TPM ought to be able to figure it out? Anakin survived catching fire, and Grevious was little more than a brain in a jar, so it’s not that improbable by Star Wars standards.
Then again, I’m the one who thinks Mace survived that fall and lost his memory. 😉