No, it isn't. Imagine someone got a developer's username and password at Microsoft. Then they logged in and managed to escalate themselves to Administrative privileges on the box that manages Windows source control via a security vulnerability. They then injected some backdoor access into Window's networking code for an upcoming patch. I would argue the chances anyone at MS would have noticed this is actually lower than compared to a distributed environment (git) that is designed from the ground up to catch these kinds of things. There is nothing about a proprietary system that makes this kind of thing any less likely.