Of course it's a tomb. You have the mortuary temple out in front of it, with texts telling you exactly what it is. It fits in stylistically with mastabas, step pyramids, and true pyramids that come before and after that are unquestionably tombs.
Old Kingdom mummies don't tend to survive, probably due to a combination of their extreme age, the still-developing mummification process, how uncommon mummification was in that period, and that mummified bodies were targets for looting given the precious objects that could be found on them and in their wrappings. That a sarcophagus is empty that has been open since time immemorial in a tomb that has been known to be looted in antiquity is utterly unsurprising.
Smart tomb robbers would have themselves buried in the tomb along with the king and all his gold, ostensibly to help him in the afterlife. Then they would escape out the back door.
Have they ever found any skeletons of these royal helpers inside any tomb?
It used to be common to execute retainers to help out the king or queen in the afterlife. Sometimes just a few, sometime hundreds. Supposedly everybody who worked on Genghis Khan's tomb was executed, along with his entire funeral procession, and all their executioners, so nobody would know where it was. It was finally found, just recently.
I recall a story where an Egyptian priest tossed the royal mummy in the Nile and committed suicide in the pharaoh's tomb, thinking himself clever for it.
