I certainly hope the takeoff data was valid starting the roll down the runway?!!
I am quite sure some extra margin was available as in low vis no reduced thrust is allowed(no ass,derate ok).
From top of my head,if its a 738 at max takeoff weight,it needs about 2400 m of TORA.
Quite sure no intersection take off are allowed in low vis and that FRA runway is longer than 2400 M plus line up distance.
No problem there...