Cannot target with card effects?

Playing dark magician girl deck against Superheavy Samurai... In my field there is “Kiwi Magician Girl“, the effect says “Spellcaster-Type monsters you control cannot be destroyed by card effects, and your opponent cannot target them with card effects”, I attack directly and he activate the graveyard effect of “Superheavy Samurai Gigagloves” which says “ You can banish this card from your Graveyard; excavate the top card of your Deck, and if it is a "Superheavy Samurai" monster, add it to your hand, and if you do, the attacking monster's ATK becomes 0. Otherwise, send it to the Graveyard.”, then he draw a superheavy samurai monster and the kiwi magician girl’s ATK goes down to 0... WHY?!