It was a social engineering attack that leveraged the device OAuth flow, where the device gaining access to the resource server (in this case the Salesforce API) is separate from the device that grants the authorization.
The hackers called employees/contractors at Google (& lots of other large companies) with user access to the company's Salesforce instance and tricked them into authorizing API access for the hackers' machine.
It's the same as loading Apple TV on your Roku despite not having a subscription and then calling your neighbor who does have an account and tricking them into entering the 5 digit code at link.apple.com
Continuing with your analogy, they didn't break into the off-site storage unit so much as they tricked someone into giving them a key.
There's no security vulnerability in Google/Salesforce or your apartment/storage per se, but a lapse in security training for employees/contractors can be the functional equivalent to a zero-day vulnerability.