This looks like rust caused by metal differential electrolysis; two dissimilar metals in contact will cause rusting. If so, this is down to the choice of materials by GM and has nothing to do with moisture ingress; you have to store a car outside for a long time before you get internal water damage - your paint will have suffered before this happens.
Contrary to what the stealership maintain, it's not 'normal', but can happen. It's not going to cause the car to fall apart, but does need to be addressed - at the very least it needs to be cleaned, treated with rust convertor and checked regularly - it's impossible to tell when the electrolysis will run it's course - it may have stopped, it may continue, there's no real way to tell...
And as others have said, it's not your responsibility, it's for the manufacturer or their agents to address.
Keep us updated please...