Aha !!!

So in the first version of Divinity II (Ego Draconis - so withhout the Flames of Vengeance add-on!!), the spell damage was ONLY related to the current level of skill itself ....
... BUT with the Remastered Version, Divinity II : The Dragon Knight Saga (on Xbox = also with the Flames of Vengeance part included), also Intelligence give extra damage to magic spells !
I'm very happy Larian make it that way ! It's more logical Intelligence boosts Magical Spells too, because Magic Spells are ALL that is Magic !
Thanks Raze & UsingBlanks for the info !