Those numbers are not off. They are tested on tracks.

You need to remove your shoe and drive with like two toes to get the speed just right, though.

Test drivers I have done this with takes off their shoes or use ballerina shoes.

Cruise control?

No you want to control the shape of the speed curve to not overshoot and not accelerate too much, when you follow the speed profile.

And keeping steady state speed is not that hard.

Hrm it is a bit funny that modern cars are drive-by-wire (at least for throttle) and yet they still require a skilled driver to follow a speed profile during testing, when theoretically the same thing could be done more precisely by a device plugged in through the OBD2 port.