Originally, the group was divided into EXO-K and EXO-M in the game, but after Kris, Luhan, and Tao left the group, EXO-M only had 3 members. Now, the game has combined both EXO-M and EXO-K into a single group as 'EXO' for their newest songs, excluding Lay.
Technically, removing Lay from the songs makes sense, because Lay is not in EXO's newest songs such as "Power" and "Ko Ko Bop". However, Lay was originally part of "Ko Ko Bop", but the newest release resets all scores for the song, taking Lay out as well.
It should also be taken into consideration that 'SuperStar SM' usually makes a separate group for new songs with events, so everyone can have fair chance at the events, but will the new EXO group stay permanent?
Did you notice the change to EXO in 'SuperStar SM'?