You are right.
The calculation for balance field considers max braking (manual or RTO) with no delay for braking or speedbrakes actuation.It does not considers use of reverse thrust:' Baseline: AFM balance field length== All engine RTO,brakes and speedbrakes only,no thrust reverse "
So ,one should follow FCTM procedure,which says : close thrust levers,aplly max brakes (or vrify RTO autobrakes) ,then raise speedbrake lever.
After that apply max reverse thrust consistent with conditions.
Failure to raise the speedbrake can give a penalty of +630' (no reverse) or +310' (one thrust reverser)