No they're making their films in the future also in UE4. From their FAQ's:
How will your movie production be impacted by Subverse?
We will continue making movies in the future. Instead of jumping into movies right away in a new engine, we chose to get some practice by using the game as our experimental lab.
We’ve already improved so much from 6 months ago, and we feel that we will improve even more during the development of Subverse. This will make sure we avoid the dreaded “first-year syndrome” that plagues 3DCG artists who are still learning a new engine.
Case in point - compare Lara in Trouble (2014) to Kunoichi 3 (2018), which was done in the same engine. The difference due to experience is vast. The experience we gain on the game, while the programmers are able help us with a lot of the toughest tasks, will translate into much higher quality
So the game doubles as their own IP as well as practice for when they get to making movies again.
And to drop another part from the FAQ's:
Is this going to be different from the flash games you did before?
It’s like comparing a Lada to a Ferrari - not even in the same league.