Hey there! I'm one of the devs working on Ambient.
To answer your specific question, nope - the server is always authoritative over state, so clients cannot adjust reality for other clients. That being said, there are other concerns that result from this model, so I'm going to transclude my answer from Reddit (https://www.reddit.com/r/rust/comments/118wlda/introducing_a...):
"By default, components are not networked to the client; you have to opt into this using a component attribute (https://ambientrun.github.io/Ambient/project.html#reference). That being said, many of the core components are networked by default - we're thinking about how to best balance convenience of development with security, and it's an ongoing discussion."