My understanding, from speaking with Marcel Aubron-Bülles, Jonathan Fruoco and Pieter Collier (the trio who had made a translation and approached both the Estate and Le Monde for permission to publish it) is that their objection is not that the translation is inaccurate as such, but rather that it puts Christopher Tolkien in a negative light whereas they feel that he is, in the French original, portrayed with far more sympathy.

I have never learned French, so I cannot speak to the subtext of the original article and so I cannot say whether they are right in their views, but I don't think that Christopher Tolkien comes across as unreasonable in the English translation (though perhaps he does appear a bit disconnected from contemporary commercial reality).