The posted video is interesting and gives some good hints. But with separating the mix from the arrangement/composing there are different opinions. Many preferring to (pre)mix the same time they using the sound. Often mixing can change the sound so they will have it done instantly and see this stuff a bit like a part of the sound. At the end of the project some fine tuning, leveling etc. but the most part is done earlier (so there is not a big mixing stage).
Anyway, it's true not to spend too much time at such early process with mixing and eventually mastering. It should sound good but must not be ready for publishing



