I mean the fact those examples contradict that particular statement, I'd say that Three Dog is absolutely misinformed on this! And if not misinformed, definitely overestimating the strength of dem Vault Doors.
I dunno really. I know three dog seems to be disliked by one and all. I reckon the doors would weigh quite a few tons and be very difficult to shift. Showing the doors laying flat as if pushed out from inside seemed strange. Maybe rats nibbled away at the workings