This is simple. If the dystopian Sentinel-ruled future is an alternate timeline then they can use any characters they want, even those dead or cured in the original trilogy.
The time traveller(s) come back in time to change an event and the result of that change could be the X1-X3 trilogy we have already seen.
But that alternate future is wide open for adding in Cyclops and Jean.
It might be the ideal thing to do if they want to bring back James Marsden and Famke Janssen.
If they instead want to introduce new, younger versions of the characters played by different actors, then that requires another storyline.
But if they're bringing back the original trilogy Scott and Jean, then an alternate future is the way to go.