I always thought psionics were completely different from magic?

Psionics weren’t effect by anti-magic measures and Mind Flayers, a purely psionic race, saw sorcerers, wizards, and such as heresy.

Did something change in the lore/mechanics? Depends on when you look.

3.5 psionics were absolutely treated as magic, with an optional rule variant to treat them as separate from magic.

— Dan Dillon 👥 (@Dan_Dillon_1) December 23, 2019

More to the point for 5e, arcane/divine/psionic are descriptors of how a creature harnesses and manipulates the magic of the multiverse in a narrative sense, but mechanically those terms are meaningless. If a wizard and a Light Domain cleric both cast fireball, one is divine and the other is arcane narratively, but mechanically and in the metaphysics of the game world they function and fail in exactly the same manner.

Why would a psionic fireball be different?

— Dan Dillon 👥 (@Dan_Dillon_1) December 23, 2019