No, they reduce all S/PDIF jitter (transport, cable etc). They cannot reduce jitter of their own D/A clock.
There is also jitter in A/D process - that one cannot be removed. Poorly digitized tapes at the beginning of CD have to be digitized again (with better clock).
There is also jitter in A/D process - that one cannot be removed. Poorly digitized tapes at the beginning of CD have to be digitized again (with better clock).