Please, correct me if I'm wrong.
1- From the "spoiler nada" call to manual brakes application five seconds went by, not ten. Why apply manual brakes before the spoiler status call?
2- The pilots "cycled" the ground spoilers armed switch almost at the same time they started applying manual brakes. This is probably because they knew braking would not be effective with no ground spoilers.
3- Autobrakes at medium is the correct (or recommended) selection for slippery runway (I learned this here).