Error is propagated to JavaScript, I tested it as described in the test plan.
Additionally documentation supports why it works: messageResult is a C++ promise https://en.cppreference.com/w/cpp/thread/promise. This type can contain either result as the object of certain type (set by set_value), or pointer to an exception (set by set_exception). Calling a get on a promise that contains pointer to an exception is equivalent to throw exception https://en.cppreference.com/w/cpp/thread/promise/set_exception. In other words if we call set_exception on a promise then calling get() on the promise throws the exception. Creating JSError and setting it in the promise happens on different thread but get() method is called on the sam thread the function executes so JSError is thrown on the same thread the getAllMessagesSync function executes.