ValueTree doesn’t rely on message-thread. It has synchronous callback behaviour (on the same thread where the changes happen), while others like ChangeBroadcaster are asynchronous, and callback on the message-thread.
This is simply a matter of definition and should be taken into account when programming.
I agree, there should be better tools (like wrappers for thread-safe access to / and callbacks from value-trees) and warnings do deal with it.