I do think that it would be technically possible to, eventually, abuse this system. The problem is, these "guarantees" create a misunderstood sense of security. They want you to believe this is "more" secure. But "more secure" than what? Competing cloud platforms, maybe.
What if someone had access to 1Password's "vaults". They would only need to phish the user for two pieces of data; the 1Password password and the account key. Then, without ever having hacked your network, they gain all your passwords.
So anyone with a back door into 1Password gets a trove of accounts that they can phish.