Mostly additive. Lucid does try to smoothen out the brake pedal on how aggressive to apply regen braking before starting to use the mechanical brake. From my experience with Tesla M3 and the Lucid test drive its still more or less regen being applied less you're doing actual aggressive stopping.
I'd say 95% of the time you'd be hitting regen, whether lifting off gas pedal to using brakes to slow then stop. Only when you're coming to an aggressive stop such as sudden maneuvers or whatnot, then mechanical brake.