It makes sense. My understanding is that, in the chain
Innuos (usb) -> U18 (i2s) -> X30 that you have,
U18 is doing the re-clocking of usb signal. If using external master clock in U18, it will help clean the output from U18 going into X30. The cleaner the output from the upstream DDC, the better (less jitter and phase noise) the signal will go into DAC. So in this case it is better off to use master clock in U18.