> this changes the cost of a block withholding attack from 50/50 pool/attacker to >50% cost to the attacker

This is incorrect, but concise. The cost to block withhold attack a PPLNS pool is exactly in line with the hashrate used to make the attack. In other words if an attacker uses 100 TH/s in a block withholding attack, they cause themselves and the pool an equal amount of damage. All miners lose out on the potential rewards of those 100TH, but so does the attacker. Meanwhile, the pool continues to pay out rewards to the attacker as if that 100TH/s was mining honestly.

If the pool pays an extra 5% to the share that wins a block, now the block withholding attack costs the attacker 5% more and the victims 5% less.