1:a usually swinging or sliding barrier by which an entry is closed and opened;also:a similar part of a piece of furniture
3:a means of access or participation:opportunity<opens new doors><door to success>
Hmm...no mention of Rocks...
Definition of ROCK (Noun)
1:a large mass of stone forming a cliff, promontory, or peak
2:a concreted mass of stony material;also:broken pieces of such masses
3:consolidated or unconsolidated solid mineral matter;also:a particular mass of it
Definition of ROCK (Adjective)
1: : in a difficult or uncomfortable position with no attractive way out
— between a rock and a hard place also between the rock and the hard place
Nothing about Rocks acting as doors. Just acting as objects of mass and obstacles of difficulty.
Besides, Dragons don't block their lair. In fantasy, the Dragon's lair only has one way in and out for its defensive reasons, and it needs to be open to ventilate stench and gasses emitted from the dragon's breath. Any savvy fantasy buff would tell you that Fire-Breathing Dragons exhale hydrogen. If it were to block the entrance to its lair/cave, it would risk suffocating on its own breath. Your argument is moot.