Loosely based on AOA is what Singer said. The writer called it "the X-Men's first scrimmage with Apocalypse" which indicates it'll take more than one movie to take him down. Its set in 1983 & it features Jackman, McAvoy, Fassbender, Lawrence, Hoult, Peters and Channing Tatum as Gambit along with young versions of Nightcrawler, Cyclops, Storm & Jean.
No indication so far that the 2023 cast will appear.