It isn't a certification or aerodynamic thing it is just ar$e covering thing.
Years ago use of flap on takeoff was recognised as increasing the aircrafts utility balanced against the almost vanishingly small increased risk of having an engine failure airborne in that configuration.
As we moved into an era of product liability lawsuites and enormous payouts that all changed.
The model 55 went out of production about this time thus it's AFMs remained in a timewarp whereas the 58 is still being built so it's AFM was updated with the new 'thinking'.