I reiterate. The inside of the ship is made from metal too. Saitama breaks the outside by punching the inside, which further proves their durability to be comparable. Narrator says "the door didn't last more than one hit either", meaning it is a surprise that the door couldn't tank a punch that was breaking the outside of the ship. This is confirmed
by this statement, which says that the ship is made of a metal that is much stronger than steel and each area is sealed with strong doors.
All of that is outright confirming that doors (I don't care whether they resemble rocks or metal) are more durable than the rest of the ship, which is 6-A.