Well did you read the thread?
Joshua gave the likely answer there.
It being their first game that player may have missed some details, but if you notice their map the
entire blast door is already open, and once that happens the shell can open automatically as well.
Above I was only referring to the normal direct mechanism for opening the shell, but technically it might happen indirectly due to a number of other causes. I shouldn't have really written "have to go through" since it's not the only way, but I had actually written a much longer paragraph to respond to you on that point with some additional architectural details and decided to delete it because that was going to just get unnecessarily complicated
You should operate under the assumption that it's related to the main door.