I put this into the same category as the "that griddle was seasoned by decades of cooking bacon so you know that burger will have extra flavor" which I think I heard the mayor of Flavor Town say once.
What, they never cleaned it?
Sous vide won't get you a maillard rxn, so no browning and the flavors that grow from that. But you wouldn't get browning in a confit anyway. So I think you are right.