Yes, they do own them. However, they own just as many or more already proven IP's they could use to fill out another park. And Universal, thanks to the success of Harry Potter and further shown with winning over the extremely protective Nintendo, has made a name for going above and beyond to work with the IP holder down to the smallest details. Don't think for a moment they would chose something like "Mortal Engines" at this point in planning over any number of other established popular IP's regardless if they own it not.
As far as the "cult status" thing... They aren't going to build something because "hey, even if it fails, it'll become a cult classic!" No singular entity has any control over what films become cult classics. Its not something that can be forced and forseen ahead of time... Its just something that happens naturally over time. Sometimes a LONG time...