I am rather skeptical as well because the fan(s) can be tested separately with electric motors on a test stand with a suitable fairing mock-up to establish the noise output of the fan(s) alone. There is no need to go to the trouble of also building a rather expensive engine core for the purposes of evaluating the fan(s) in early stages of development and doing so would lend the ability to better characterize the fan(s) and either tweak the fan design to the expected engine core performance or tweak the engine core to match the fan demand.
A risk mitigation policy would be to do such a test early on to prove the previously demonstrated problems, particularly sound output, had effectively been dealt with.
I don't doubt they can build an engine with the fan(s) on it - I have doubts it is anywhere nearly as quiet as they claim. Saying it is quieter than low-bypass turbojets engines is avoiding what could be easily demonstrated.
My aerospace degree was not in propulsion - just typical aircraft aerodynamics - so I don't have an estimate for shock wave formation at the blade tips, but I recall this is the effect that limits the speed of conventional propellers and the efficient speed of the aircraft.