I think you're missing their point. The time it takes to crack a key is given as an average. In reality, half of all 64 bit keys are crackable in the same amount of time or less than what it would take to crack a 63-bit key on average. So if they are saying that it's feasable to crack any 63-bit key in that timeframe, then it must also be true that it's feasable to crack around 50% of 64-bit keys in the same timeframe. Clearly that's still unacceptable.