Actually, I heard Phil in an interview say that the title came from a guy who worked at the recording studio who had a stutter or speech impediment, so he couldn't say the word studio correctly...it came out sussudio.
The song title is NOTHING! PURE NONESENSE! In an MTV interview Phil stated so. He penned the music first, then the lyrics. Sussudio was a nonsense word he used as a sound while writing. He just left it there after nothing else fit. Ever meet someone named Sussudio? And, how rude but to rail on the guy at the recording studio. Good way to get your session recordings messed up...
According to Phil Collins, when he was on VH-1's Storytellers, Sussudio is an imaginary girl's name. The song is about having a crush on someone when you are young. He used Sussudio as a name to encompass any girl.