Is it really a GE requirement?
Yep, with the CF6 engines on the plane I fly, if the OAT is 3degC or below run up to 60% N1 for approx 30seconds to confirm stable operation.
Same plane but with RB211's,
Run up to 60% N1 for 10 secs to confirm stable operation.
There is also a periodical run up requirement for each type of engine for ground operations in extended icing conditions.
Regards,
BH.