Earlier quoted context omitted.
Publishing your vault of passwords on a public GitHub repository seems like a pretty bad idea, no matter how well you trust the client-side encryption code. I'm no expert on the subject, but I suspect these password managers use a sophisticated mechanism of authentication (for accessing the vault) as well as encrypting of the actual contents of the vault. The effect of this means that Bob's encrypted vault cannot be…
And why do you think that authentication + encryption is going to be more secure than encryption alone? Either way, there's a secret. If you know the secret you can access the data and if you don't you can't. The UI/UX trappings of the mechanism don't change this fundamental dynamic. What does change the dynamic is if you allow a third party to control the code that you run.
Further, allowing anyone to download my encrypted vault just feels really uncomfortable and unnecessary.