So is this a semantical argument about the difference between "The Movie" and "a movie"? Because if the line is as important and well-defined as you seem to think it is than there are movies literally titled [Franchise]: The Movie that apparently don't qualify as The Movie.
Just because Back Cover isn't a recap of KHX doesn't mean that it doesn't function as an introduction to the Age of Fairy Tales. It still introduces Daybreak Town, the Foretellers, Ephemer, and the Dandelions. It even directly adapts a few scenes. The only element of KHX that seems to be important to the saga as a whole (as opposed to the Chi sub-series) that didn't make it into Back Cover was Skuld (assuming that she's Subject X) but I've seen people theorize that it's because her design wasn't finalized while the movie was in production.
There are probably a few reasons why Back Cover wasn't a direct adaption of KHX. Not wanting to put a canon face on the customizable protagonist and wanting to put more focus the Foretellers without disrupting the story flow are probably some of them. Another would be the plot at the time not being very developed, so they had to add more material for more than a short. And of course, SE wanted people to play their gacha game so they weren't going to make a movie that functioned as a complete substitute when a glorified commercial would be more profitable.
So Back Cover 2 would naturally have new material, but there are parts of KHUX that are much easier to adapt under my theorized parameters. I've often wondered if one of the reasons why the player spends the majority of the time far away from the union leader plot was to make any potential adaption easier. But KHUX no longer being an active project is a real potential killer for any potential movie.