This is one of those things that's assumed as true. The race authorities are always designing the parcours to get a French winner and the commissaires are always favouring French riders and particularly hard on non French riders. But it never seems to actually happen.
Except maybe for the mystery absence of TV footage of Bardet making up a remarkable amount of time in one stage a couple of years back. But that was the TV not the Tour authorities.
I really want Thibaut Pinot to win. Compared to that, Bardet is a safe bet.