Every 3 dB is a doubling of actual volume. So 15 dB quieter means 3% of the original volume. But that sounds hard to believe since even electric propellers are very loud, and small ones are louder than large ones due to the necessarily higher RPM. But for sure it would be much quieter than a helicopter.
eVTOL sounds dubious to me but there are a lot of major companies pursuing it so there must be merit to it. I believe they have no auto-rotation capability though.