Is flap-asymmetry protection on airliners still active even on the ground?
It is active whenever the flaps are being moved by the hydraulic system on the 737; it isn't available when using the backup (electrical) system.
Allowing asymmetric flap extension before takeoff could have consequences as serious as asymmetric extension in flight. Plus, tying the protection to an air/ground sensor is one more way that the system might fail.
Evacuations are pretty rare events; the chance of having one along with a malfunction as described in the OP is extremely remote. Such an unlikely occurrence doesn't overcome the reasons to keep asymmetry protection available full-time.