Properly, "box". There is no evidence a body was ever in it.

Is a car only a car once somebody drives it?

Does a thing sorta car-shaped without wheels count as a car? This thing never even had a lid. (You could call it a car, I guess... Rectangular, and you can sit in it.)

Many people hope a legitimate sarcophagus, with an actual mummy in, might yet be found.

At which point of building a car separate parts become a car? And during maintenance (lets say engine replacement) is a car still a car? I'd say it is if meant to be used as a car again.

Right. So, is a stone box in a pyramid chamber that there is no evidence ever held a corpse, or even had a lid, and certainly will not have a corpse in the future, really a sarcophagus? It seems to depend on overconfident assumptions.

Given their experience, a decoy that looks like a looted tomb would just be good planning.

It looks like all the other sarcophagi found in other Old Kingdom tombs. So no, "box" isn't specific enough.

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.

Tomb robbers have not generally been given to making off with sarcophagus lids.

Tomb robbers are given to smashing anything that gets in their way.

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.

Smashing is one thing, carting off is wholly another.

It quacks like a sarcophagus.

Thanks for pointing this out!