Ships like that are typically optimized to look small from "low" angles, ie from the perspective of other surface combatants and sea skimming anti ship missiles. The large flat surfaces are not so much used to reduce RCS by themselves, but mostly to reduce instances of "corner reflectors" like hatches and exposed cranes the like, which can have a RCS many times larger than their physical size due to their shape.

See also the "Reduction" section on Wikipedia in the article about Radar Cross Section: (https://en.wikipedia.org/wiki/Radar_cross_section#Purpose_sh...).