I dont know, maybe they'll explain at some point. Everyone else clearly thinks he is dead. Magneto is gonna be big mad when he learns Charles is alive.
Charles was never dead in the original series. It came really close to him dying…. If not for Lilandra coming along in the finale and saying that she can save him, but she will have to take him out to space.
During the first couple episodes of the series , I thought that the whole “Charles’s dead” was a cover-up by the X-Men and Magneto.
But now, I am starting to believe that Charles decided to just YOLO it with Lilandra and don’t tell anyone back on earth that he’s ok.