Are you sure they work on the Engine Core themselves, looking at all the missing things i would say not yet and looking at still some performance related things i would say it still is heavily Blueprint.
Definitely this project can't be compared with for example Kingdom Come Deliverance in those regards or a full Self Adapted Unreal Engine Branch yet like most AAA Unreal Engine License using.
It shows the core of Epics Branch Advances though nicely
But they have a really great Art Team Core and that really shows so far im a big fan of the Maya based Animation work results and the World and Character Design and some of the really nice working Mechanics so far.
You don't Design a Open World in 1,2 or even 3 years especially not with a limited Team and make it a coherent visual experience with some Artistic Language behind it.
See how Long Square Enix needs with 100 of employees several teams working on Final Fantasy or Rockstar on GTA and Red Dead Redemption or Bethesda on Fallout, Skyrim or Ubisoft Teams (those are many studios working together and sharing knowledge) for Assasins Creed,FarCry,WatchDog, so in those regards Wild Life Advanced visibly and is one of my overall favorite Independent Unreal Projects that tries todo Open World which you overall won't find many trying it on that scope at all, Open World is massively complex the most complex task you can put your project on you must have real guts todo it to practically proof a Engine Core your Team and the whole Workflow is ready for it.
Yet the most successful was Kingdom Come Deliverance but that is a really well trained Team that wanted to go Independent like CD Project Red and no new Team that is coming to life like we see in Wild Life on a Engine that was some years ago on it's own base unthinkable to use in a Open World Production and even more worse then something like Creation Engine Bethesda uses for virtually Decades now with so many refactoring to make it all work, you might add (more or less) and holding it with a lot of glue together before moving the whole Team to a new Core finally someday (probably one of the most advanced core of them all by that time, whenever that transition should finalize)