they also say
Suggest you run a search on derate .. plenty of threads around.
Concern is that
(a) ATM is just a matter of taking off at less than rated thrust. All data remains OK .. provided that the reduction still gets you off the ground etc. in accordance with all the various rules
(b) derate, functionally, is the same as bolting on a lower max (rated) thrust engine .. and ALL the certification bits and pieces are tied to that lower maximum thrust .. including things like Vmca/Vmcg (hence the reference to directional control). So, depending on the actual engine installation, potentially you could run the throttles up to a thrust setting which could embarrass you at lower weight speed schedules ..