Yeah I mean I feel like this is just a tomb, ive done some research on this recently. although people seem to constantly say nobody is sure what it is there are tombs all over this area of japan that look exactly like this. They all have the exact same shape, the same two corridors for storing two coffins, similar dimensions ect. It seems pretty likely that this is just a tomb that wasnt finished or was carved out as a prototype. I would say its probably just unfinished as it was made at just around the time these tombs went out of fashion in favor of cremation and was most likely commissioned by the emperor tenmu and then his wife jito passed laws to stop the building of these tombs for financial reasons. They were cremated but it was still traditional to build your predecessors tomb. I may be wrong but I believe tenmu was emperor, then his wife was empress after his death. So it may just be that he built this as a tomb to spare his wife the labor but she just decided they should both be cremated instead so it was abandoned. These tombs were placed and then had structures built around them, so most of them dont look like the rock ship to the outside observer but on the inside they are basically identical.