Blocking distortion is caused by the coupling capacitor taking a long time to discharge once grid current has stopped flowing.
So, as far as blocking distortion is concerned, the "R" in the RC time constant formula is the shunt resistance from grid to ground. Not the stopper resistor, which is in series with the grid, and therefore can only have an effect while grid current is flowing.
Yes, a capacitor across the stopper resistor would stop it from filtering out RF and snubbing oscillations. I guess Dumble was only interested in the tonal effect of it, which is to reduce the treble rolloff caused by an oversized stopper resistor in conjunction with the Miller capacitance of the tube.
12AX7 type tubes usually won't oscillate even without the grid stopper, and RF only needs to be filtered out of the first stage, so he would have got away with it.