I think it just means that the voltage will increase up to "almost" 9V.
The capacitor will never quite reach the battery voltage, but it will get close.
I think, in order to get up to 300V you will need something called a "charge-pump".
I'm not quite sure how it works, but I think it converts a DC voltage to an AC voltage -
then a transformer can be used to change the AC voltage up or down -
finally, the AC voltage gets converted back to DC.
There are also some IC's that will boost an AA (1.5V) up to 3V or 5V.
(such as the MAX756)