The only reason I connected the pressure washer to my block directly was because in my case I couldn’t get any flow out of the back drain hole with even a 1/4 inch rod pounded into the hole and wiggling around wire hangers
Yeah if it’s flowing you can skip that step

I wanted to know I could drain the rinse water and subsequently the used evaporust from my block before I put it in
So adapting the pressure washer directly was required
And when the block is empty the action of the pressure washer was much more dramatic coming out the top of the radiator than you could get with just a garden hose
It really froths up the water and makes a lot of turbulence

But anyway there are many ways to fix this issue

Washing soda won’t do anything agains rust in a block
That’s just gonna clean oil residue and easily flushable particulate that would have been rinsed with hot water
-s