Yeelight is just a startup marketing under the Xiaomi brand and is promoted by them, it's not technically Xiaomi. Same as with other branches of products (e.g. the vacuum robots).

It also happens that Xiaomi later releases products which actually compete with their partners (in this case yeelight and their bedside lamps). Also the whole Yi Camera brand. First marketed with the help of Xiaomi and now a competitor to the Mijia brand (Xiaomis own brand).

(Citations and more insight needed, that was just from memory).