I would translate the part about being unable to be "affected by spells" as meaning it can not be subjected to an effect caused by the spell. Blocking line of sight is a trait of the Wall, an inherent property. It is not an effect caused by the wall.
In other words, it is the wall itself that is subject to this trait, being made opaque so you can't see through it. It is not actually doing something to the creatures in question. They just have to deal with the properties of the wall, similar to dealing with an Invisible Stalker or a Flying Creature. Such traits haven't actually affected your creatures to restrict their ability to attack, they've affected the defending creature to increase it's defense. The wall's trait doesn't reduce your ability to see, it increases it's difficulty to see through it.