No you are thinking of a terminating impedance nothing to do with what is being talked about here.
It may very well have an impact on why the video signal is degraded. If the multiplex chip has any significant resistance across it, then in effect a voltage divider may have been created. On the video capture cards I have, the coupling capacitor that captures the video signal is up stream of the 75 ohm impedance producing gizmo. If the multiplexing chip has signifiant resistance across it (I think the 4051 chips are ~100 ohms), then the signal voltage at the coupling capacitor may well be significantly reduced.