Interesting you say that you would not assign a user db_owner, because the ESRI documentation says: "Alternatively, the upgrade can be run in SQL Server by a user (other than sde) who is in the db_owner database role or sysadmin fixed server role. This does not apply to database services." Found here.
I assigned myself db_owner because I am going to be controlling the database, its users, permissions, etc. That way I can add users/assign permissions, assign roles, etc, all without having to sign into the SDE user.
So while yes, the sde user is the geodatabase administrator and is used to admin the geodatabase, the db_owner is on my user account giving me full control of the database.
Also, in step 5, sysadmin is different from db_owner, if that is what you are talking about.