Seems reasonable suggestion to me when starting a car ,manual or otherwise, to have your foot on the brake.
My reasoning is as follows;
To start a manual car for example you need to push the clutch in.
If the handbrake is on (assumption) and not working properly the car may roll.
If the handbrake is off and car in gear, the car may roll when you depresss the clutch
My attitude is simply that if you are not moving then for should have the brake pedal depressed unless you have engaged the handbrake and tested to ensure handbrake is working correctly.
This is, of course, ignoring the situations that may arise when people think they have disenged the gears and start car with foot off the clutch......atleast with your foot on the brake, the car does not move as much as it would with no foot on the brake
and a final reason for depressing the brake pedal when starting....
you make sure someone has not cut the brake lines (as always happens in the movies)