First: "talk someone down" is to calm them down (it comes from literally talking to someone until they are convinced to come down from a high place and not commit suicide.) Where as "talk down to someone" is to speak to them as if they are inferior. In the song, he's asking to be "talked down" because his misses his former lover and doesn't feel emotionally stable without them, so he wants them to return so that he feels normal again.

"To talk someone down" and "to talk down to someone" are entirely different. The first meaning you read is actually true: you talk to someone with the aim of making them come down from a high place. In Troye Sivan's lyrics, he means "come over here and talk to me so that I calm down". You'll see the rest of the lyrics refer to his wound-up and anxious state of mind.