On a rejected take off, the speedbrakes must be deployed BEFORE reverse thrust, according to mr Boeing.
This is to prevent assymetrical reverse thrust (in the one engine scenario or the reverser u/s scenario) with the speedbrakes down which can cause control difficulties.
This is the reason why the speadbrake is not armed for t/o. It should be done manually, after thrust lever closure. Then reverse thrust follows. That's Boeing SOP anyway.