You are right:Schwants never used the rear brake but not everyone is riding his Suziki which had a rather special chassis geometry.
"You snap the throttle shut and bang down the gears, causing engine braking to break traction",
"Banging down the gears" the way you think causes the right hand to variate pressure on the brakes, and the front suspension to pump up and down, one of the first things i learned at Carol Racing School.
You'd get all the gear down for one specific corner at once and keep constant pressure on the brake lever.
Transitioning from braking to engine brakes has the same effect but if you have started your braking from the rear, when you hit the front, both your suspensions are already compressed, meaning you limit the weight transfer.
Using the rear this way helps getting the bike more stable, building up temperature on the front tyre before using it, lowering the bike of both its suspension, so you can brake HARDER and enter the corner FASTER.
If your chassis is designed/set up for acceleration you will find it easier to put the power down because there is less adherence and the yre is not going to let go so abruptly, while you'd have carried all the corner speed the bike can take at the apex. Simple.