Luke is definitely standing in front of a very old grave.
I was wondering about that myself. Seen it 4 times and still couldn’t quite tell if it was a grave or just a crooked rock.
Notice that the third time. Has to be a grave, right? It’s the final scene of the movie and a big set up for the next one, you have to imagine they spent a lot of time making sure the setting was exactly right. I only buy it’s a crooked rock if that crooked rock is actually a natural part of Skellig Michael, and even then it’s possible that location was specifically chosen because of the rock.
I mean, it does seem silly to put so much importance into a rock but Luke seems to be looking right at it. Must be a reason for that, no?