The card "notes" mean a spell-type card being played, not the type of damage. The "12" comes from adding up the values in the example. The card text does not mention 12 spell damage because there is not 12 spell damage. The example
does add up to 12 damage being done (in the example) by the spell card idea being proposed.
Hopefully that helps clear up this recurring "spell damage base" mistaken inquiry.