No, this is the fundamental tradeoff they made to make the Bronze'd Rust easy to use:
> Rust permits only one mutable reference to a value at a time...
With Bronze, mutation is permitted through all references to each
garbage-collected object, with no extra effort. A key tradeoff is
that Bronze does not guarantee thread safety; as in other garbage collected languages, it is the programmer’s responsibility to ensure
safety.
Allowing mutability anywhere is what fundamentally makes Bronze easier to learn, and more error-prone.
The paper's author has already explained to you how thread safety can be achieved in a production-ready gc. This is not something that is particularly difficult to engineer and is quite orthogonal to whether one chooses to use tracing gc, ref counting or Rust-like borrow checking.
> Rust permits only one mutable reference to a value at a time... With Bronze, mutation is permitted through all references to each garbage-collected object, with no extra effort. A key tradeoff is that Bronze does not guarantee thread safety; as in other garbage collected languages, it is the programmer’s responsibility to ensure safety.
Allowing mutability anywhere is what fundamentally makes Bronze easier to learn, and more error-prone.