It seems to be more than a theory. The first teaching of Islam is -- sorry, my spelling will be off, "Laa 2ilaaha [except] illa
7al 'l-[2i]laah, mu7ammada(n) rasuulu 7al 'l-[2i]laah i". (the "i" at the end is correct grammatically, but no one says it.) I forget the word for "except". Being a well informed person, I have known this statement in its English translation, "there is no god but God, and Muhammed is the messenger of God". But then I saw the inscription on a coffin in a photo that was published in Time magazine last summer. The deceased was the victim of a suicide bombing in Iraq. Anyway, that's how I have come to believe that "al laah" really does mean "THE God". Now I don't know, maybe the original word 2ilaah has lapsed from use except in a few set phrases such as this one.