I always got the impression that what was meant by that quote was that Simon didn't want his opinion to affect Adele's music. Right after she says that, she says, "He's fine with it." I believe she was referring to the fact that she wrote about their relationship, and he was telling her it was alright and that she didn't need his approval to write about him. Does that make sense?
So... Without sounding stupid, "I Miss You" is basically about Adele being, for lack of a better word, thirsty?
