Meteor is a spell in FFVII, just like it is in FFII, FFIV, FFV, FFVI, and prettymuch every other game. Holy is a spell, too. It's just that, in FFVII, they were directly tied into the plot. Meteor is even called the "Ultimate Destructive [read: Black] Magic" in the game. So, no, Sephiroth didn't summon Meteor, he cast Meteor.