hp doesn’t have to do with it …gear ratios do.
shift points will vary from vehicle to vehicle based on transmission and differential gearing…which varies the shift points based when you look at mph. Even then, shift points can vary on your rate of acceleration.
Even the shift points within the same vehicle will be different whether your accelerating rapidly or slowly. Some shift points are set electronicly, some through engine vacuum…etc.
if you’re interpreting “shift point” as the engine redline, and not when the automatic transmission actually shifts the gears. Redline can be effected by HP, but has more to do with the stroke, displacement, Rotational mass, balance and component strength within the motor.
tell your dad he’s wrong and that you will be smarter than him when you grow up.