Another possibility is seeing Anakin in a Force vision. They did almost use a ESB Luke double for Rey’s vision of Bespin in TFA.
Isn’t that just crazy that this image got leaked. I’m a bit surprised we at least didn’t see hear Luke and Vader’s sabers clashing as the sounds echo down the hallway, even if we didn’t see them fighting behind Rey. I still think they should’ve added a young Ben Solo looking up at Snoke that Rey sees down the hallway like it was described in the novelization.