The following is a paraphrase from a Quora answer. Sadly, the source has since left Quora and I've been unable to locate it. Here's what I have from memory.
French military planners did not believe that the Ardennes was impenetrable. Tanks were used in WW1 and they were considered a grave threat, the whole point of the Line was to defend against a mechanized assault force. The battle plans they drew up had a variation extending the Line to cover the forest. It was dropped when a deal was reached with Belgium so that they could cover that part of the defense. Years later the deal was forgotten and so the forest wasn't defended.
A French division was stationed at the forest but it was recalled for cost concerns. It wasn't a failure of military planning, it was a failure of the government to follow through.
French military planners did not believe that the Ardennes was impenetrable. Tanks were used in WW1 and they were considered a grave threat, the whole point of the Line was to defend against a mechanized assault force. The battle plans they drew up had a variation extending the Line to cover the forest. It was dropped when a deal was reached with Belgium so that they could cover that part of the defense. Years later the deal was forgotten and so the forest wasn't defended.
A French division was stationed at the forest but it was recalled for cost concerns. It wasn't a failure of military planning, it was a failure of the government to follow through.