Yes, exactly. The aircraft is navigating using the FMC position.
So if that position is in error, then the navigation is that much in error.
However, as you point out, with multiple sources, that error is likely small or essentially none with GPS. And there is an alerting/monitoring feature of the FMC that calculates if its possible navigation error will exceed the tolerance for the airspace or the approach being flown.
Last edited by bigduke6; 10th Apr 2020 at 13:57.
Reason: spelling/syntax