From dark memory [big buses] here; Derate = Vmcg variation. Flex= balanced field length. For example, if taking off from a short runway, takeoff performance may be improved by limiting to a specified derate level and thereby reducing required Vmcg and associated length requirements... Usually it was the A340, rather than A330 with runway surface contamination I recall.
Could be wrong, but hope that helps.
In any case- well done CNS ATC and no smear against the crew!!!