It is realizable,
they can just make some Ue3 servers that stops at the last Ue3 patch,
kind of a "Blade and Soul classic"
Also i am not sure why is Ue4 looks so ugly,
it was supposed to be looking better in previously released trailers,
So the second possibility is that they can upgrade the Ue4 to the advertised trailers quality..
or use Ue5 to achieve it..
You cannot just downgrade the graphical quality of a whole game..
it is disrespectful for the artists that made the game, and the players that paid for it..