SpongeBob was pretending to be Squidward because the actual Squidward was cleaning his house. The opposite of clean is filth, therefore he was actually ruining his house. SpongeBob usually wrecks his own house, so Squidward was supposedly pretending to be SpongeBob. However, when SpongeBob was pretending to be Squidward, he was acting like the real thing. Therefore, he was actually acting like his normal self. Patrick was actually doing it right when he was singing "I'm Squidward! I'm Squidward!"