Interesting, I've driven in 15 different European countries and found France to be one of the easiest and most chill. I mean, on the highways and city streets, anyways -- not so much on the farmland single-lane roads that shockingly have a speed limit of 90km/h lol ... but regardless, the "people merging in from the right have the right-of-way" actually makes sense to me since they're engaging in the most "high-pressure" action, while those of us strolling along on the highway can just adjust our speed to give them space, or change lanes ahead of time as needed.
I don't think they're talking about merging on a highway?
Sounds like they're talking about the fact that at an intersection, unless signaled otherwise, the people coming from the right have right of way.
People merging have right of way only on the parisian périphérique. On all other motorways in France, the merging cars must yield.
It is common courtesy to move over or match speed so they can merge more easily, but that's not the law.