I'm looking at the following quote, which would seem to indicate that, if the elements had been available, he would have done it in 4K (emphasis added):
It doesn't sound like Jeunet is saying that he is against 4K in general, since there have been releases of some of his other films on the format. Rather he is saying he believes the benefit of doing a 4K upscale for this particular film, that was finished as a 2K, would be negligible.