The guide is wrong.
It tells you to connect the batteries up in parallel (only giving you 9V) where as the standard connector connects them up in series (giving you 18V).
Im not sure how much of a difference it will make running it on half the voltage, it will probably be slower and less battery efficient.
Where the guide says connect red to red and black to black, in fact you need to connect one red wire to one black wire (leaving you with one red and one black wire, to solder to the crimp).
John