The network stream is buffered to RAM and that memory is then fed to the DAC based on the DAC’s clock. That RAM could be thought of as equivalent to some external source connected by S/PDIF or AES and thus it benefits from being synced to the DAC’s clock.
The clock used in the Bartók is excellent in its own right, but an external master clock provides a better reference. It’s simply a matter of improved performance rather than any sort of comment on the quality of the DAC’s clock.