The problem seems to be that the scratches are never on the inside.
Most if not all modern units are now installed from the outside. And the outside of the glass is where all the handling, pushing of the pane takes place. Along with putting a hand on it while you belt the beading in with a rubber mallet.................
That along with the fact that they seem to get covered in silicone and general dirt, that I've then seen builders take a rusty blade too.
This is where the damage is done.
A wfp brush with water running will never scratch glass.
You would have to trap something harder than glass (like sand or grit) onto the glass and drag it across the window.
The water washes all the grit away so that doesn't happen. Only time I've seen wfp scratch glass is when the stock comes in contact with the glass.