This warning was printed in the older manuals .( I wondered if the brush was leaving micro groves or scratch's in the barrel ? This would then just add to the fouling )
You can quickly clean a tikka just using a patch .and for heavy copper fouling I have used JB with a patch .
I find that once the barrel is seasoned the few stokes with a patch will indeed get the fouling out , and at times used a foam bore cleaner to prove it .
I have two T3's over years,that never seen a bronze brush and they shoot groups.