OK.Makes sense to me,I couldn't make it work in my mind when he told me but I knew the man well and trusted him.
He could have left out that the brush would need to be adulterated with foreign grit,dirt,etc. and any oil at all from previous use would surely attract some crud.
Also,it could be that he would know the bullets would match but he couldn't use it in a court case after the slight changes after a (dirty) brass brush had been used.
Even a tissue has dust on it that will scratch the lens on optical equipment. That's the best example of how microscopic dust and grit is all over, that I can think of.
The US Mil has brass brushes with steel take down rods and steel screw mounts for the brush. I polished the up very well, because otherwise they'd tear the bore and crown to shreds.