He can def use more than just lightning. So that is just his cover story? or can all mages use some shared pool of basic spells like the dispelia and healing magic
Yes.
They can use what's called generic spells. Which include heals, buffs and item bag and what not.
But what they are talking about is what element he has. Each element is associated to a god.
And his is purple lightning from amethyst the god of humans. Which is rare.
Some other magic stuff.
Mages have spell slots, which dictates how many spells they can keep up at a time. Buffs are single target, continuous spells and needs to be maintained.
Mc got 7 slots, so he can use 6 on buffs and still cast attack magic.
Then there are magic techniques/ skills. which use magic, but isn't a spell.