I believe that this is more of a prequel than a reboot. It takes place during the era of the Cuban missile crisis, while the main X-men trilogy took place in the "not too distant future," so we're talking probably 50 years later.
The nice thing about The X-Men being such an ensemble universe and having so many characters is that they can go back 50 years with some characters who are established as having a long history together (Prof X, Magneto, Beast & Mystique) without having to rewrite established continuity to bring back the rest of the heroes from those movies 50 years too early. The comics provide so many characters that the other movies never touched on that a young Prof X can have a whole different team in this era.