Is that what he meant? I took his answer to mean if the target line of the spell card is valid for the intended target, it's a valid cast. Take hidden enchantments, for example. You can still dispell them without "seeing" them. Maybe a mage can detect the pattern and dispell that type of magic even if it's on an invis creature?
Of course, maybe he did mean his answer to be target line "of sight" and I'm a crazy person.