There seems to be a belief amongst many that reboot with brake pedal might reset more than a reset without. Opinions are divided, and without any official guidance, who knows?When Tesla remote diagnostics tell you to reboot the car they will specifically tell you to put your foot on the brake. I specifically asked why and got it was a safety measure. Obviously one Tesla answer does not mean that it is true, but it doesn't do any harm and if it might stop your car from unexpectedly moving, I cannot see why you wouldn't do it especially as its such a simple safety measure to take.
I doubt there's any benefit in holding the buttons down after the screen goes blank. However, if holding the brake down really does cause it to reset additional hardware in the car, then it's quite plausible that you do need to hold the brake pedal down longer, as it's probably detected as the computer boots up again and then resets hardware during the boot sequence.
But as with everything, who knows for sure?
EDIT: If you're in park then the parking brake will have been automatically applied so it shouldn't be possible for the car to move.