I think that's pretty presumptuous. My experience is that rust's checking is great, but much easier and safer concurrency can still be enabled by some work with data structures and program architecture.
IMozilla engineers have specifically said Stylo (Quantum Style whatever) would be impossible in C++, because they actually tried it in C++. Presumably it'd be the same with WebRender.
Impossible is a strong word. Architecture makes a very big difference. They can claim it is impossible, but it really doesn't make sense. I'm surprised anyone would just take their word for it.
Trying to use raw threads and ad-hoc futures is going to be difficult, but fundamentally concurrency is about separating data by dependencies.
Dependency graphs that pass data around combined with lock free data structures can be used to isolate parts of the program so that dealing with concurrency is one generic part of the program.