Those skeletons were not Owen and Beru. Both were a lot more formidable than their preferred modest lifestyle would indicate. They eliminated the first two Stormtroopers to arrive and stole their armor. Owen burned the bodies, and Beru sent a phony report saying they had interrogated and killed two civilians, but found no droids. Owen had already overheard Leia’s message and deduced Obi-Wan would be taking Luke to Alderaan, so they planned on reuniting there, but lost track of them when the planet was destroyed.
Or, for a darker ending, they ended up landing on Alderaan successfully, but immediately after they landed the planet blew up. All that work to fake their deaths convincingly, and they ended up dying a few hours later anyway.