I did a little research, and although some tombs did in fact have true working doors, the majority had a series of false doors, used as a place to put offering to the gods. In tombs, the false doors were placed on the western wall, and were elaborately decorated with inscriptions about the person buried there, along with details about the gods and the process to the underworld. Typically carved out of stone or wood, these false doors may've been broken down by raiders, and could have possibly appeared as purposely built doors to early European archaeologists. These false doors were very realistically designed, often including door jambs and other accessories. The most tricky would have false passageways constructed behind the doors, with a small window to show this passageway. The doors definitely had a religious function, but there is speculation that there was also a function to deter raisers/looters. First used during the third dynasty of the old kingdom, the classic false door was adapted, with the classic series of door jambs. And like many ancient architectural features, the false door influenced many Bronze Age Mediterranean civilizations.

Sources: https://www.britishmuseum.org/explore/highlights/article_index/f/the_false_door_in_ancient_egyp.aspx

The image from that thread doesn't show the door to the tomb, but rather one of the four nested shrines inside which King Tutankhamen was buried.

King Tut's tomb did have an entrance/exit (after all they had to get his body in somehow!) but this was sealed with anti-looting wax seals (which Carter discovered intact and buried with backfill. Sometime in antiquity, thieves managed to break in, but only a few items appear to have been taken.