Hydraulic disc brakes are not overkill with a bike of that mass [0] and power. Disc brakes are very common mountain bikes or any type where there is a lot of braking as caliper/rim brakes can overheat and pop the tires, so this unit should definitely have disc brakes. Hydraulic is better than cable/mechanical as hydraulic can generate more braking power, have better feel, and stay in better adjustment. The only downside vs cable+disc is cable is more repairable in the field.

[0] (overall specs indicate a lot of weight, Rivian are not proud enough of any lightweighting to even print the weight, and their autos are also very heavy, indicating a lack of lightweight engineering discipline in that shop which may carry over to their other mobility solutions)