For years, Smule has been working fine with analog headsets, but lacked proper support for external third-party hardware connected via Bluetooth or the lightning port of iOS devices. The latest 6.4.9 update improves that.
You can now connect Lightning/USB microphones and audio interfaces to your iOS device without routing the audio through other apps like GarageBand first. This kind of hardware is now natively supported and you also get proper monitoring this way.
In addition, the support for Bluetooth headphones/headsets was improved. This is especially usefully for recent iOS devices, which come without a headphone jack. Note that this improvement only covers listening to Smule performances. There is still no official support to record from Bluetooth devices.